"Are you mad?" is a phrase Jenny Smith is well used to hearing when she tells people that she exclusively fosters teenagers. "But in all the years we've been doing it, we've never had any abusive behaviour in our home," she says. "Far from it, they are very respectful and often do very well. One asylum- seeking boy who came to us at 14 years old is now doing really well at Kingston University and is very much part of our family. Other times, the rewards are smaller. The boy we've got at the moment, who was a young offender, has managed to keep from reoffending on the scale he was for 14 months. There have been hiccups, but he's moving forward. I'm really proud of them both."

Smith decided to foster when she became increasingly sad about seeing so many "lost" youngsters. "A lot of them have had a really bad start and don't have role models. I knew that with my parenting experience, I could help. I'd also had problems of my own as a teenager and the raw emotions that I can still remember from that time are almost like a resource for me now."

All the young people who have been through her home keep in touch with her. "They pop round or come for dinner. Many bring their own children. They see us as their family. Sometimes you still need to offer them support years later. One guy who lived with us some time ago has just been sectioned and I go down to the hospital to see him when I can."

Fostering teenagers is always a bumpy ride, she admits. "With the boy I have now, he's got a mother and stepfather who are very anti-social services and authority and I try to explain to them that he's using this placement to do positive things like go to school, but they won't listen. It makes it hard for the boy."

Although Smith has managed to avoid it, some foster carers of teenagers do experience damage to their homes, aggressive behaviour, stealing, temper outbursts or running away. Others find themselves having to make regular trips to the police station or trying to persuade a school to take back a youngster they've expelled.

But foster carers still come back for more – many wishing to specialise in this age group, says Kevin Williams, chief executive of the Adolescent and Children's Trust. "Teenage years are difficult enough for anybody, but when you consider that 70 per cent of children come into care because of neglect or abuse, you can see why this group of youngsters' behaviour can be particularly challenging. Add to this the fact that many of them have had multiple moves and it's even more understandable. But what this also means is that foster carers who are firm, consistent and caring are often able to bring about really positive change."

Williams agrees that rewards come in different sizes, often small chunks. "You might see a teenager achieve at school for the first time. Or you might get a very introverted teenager who develops a level of trust and self-esteem for the first time. It's often simple things that foster carers remember – seeing the teenager laugh or smile with ease. One carer told me how she knew she'd made a major breakthrough when a youngster was able to give her a genuine hug."

The change can be temporary, adds Williams, with many youngsters regressing into previous behaviour even when they appear to have moved on. Foster carers need to be aware that real progress tends to happen in fits and starts and sometimes the impact of your work won't even manifest itself until years later.

Williams says foster carers – whether they take on teens in emergency situations for short periods or for years at a time – often liken the work to dealing with the "terrible twos". "It's about individuals trying to move towards independence and it's your job as a foster carer to provide boundaries for them to push against, but within safe limits. Often these youngsters come from a background with no boundaries, so it's quite a challenge."

For Stephen Stuart, the work is all about raising self-esteem. "We have a 15-year-old girl with us whose self-esteem was non-existent – so often the case. We have worked hard to build it up through introducing potential friends and making her feel one of the family and she is really getting there and it's lovely to see."

Meanwhile, Pam Batty points out how some teenagers can progress leaps and bounds against all odds. "We had one asylum seeking youngster come to us in 2004. He couldn't speak a word of English and had a difficult start. He's now been elected top boy in the college and has won numerous awards and certificates."

Some foster carers, such as Jim Bond, take on teenagers through a specific initiative. "I work with males who are 14-plus and it's a scheme that is supposed to be a 13-week placement – although it often winds up being more – where it's my job to assess their needs."

The youngsters tend to be at the more challenging end of behaviour, but Bond likes it that way. "I've spent long hours in police stations and in meetings at school. Then there's the wear and tear – I've gone through five washing machines and three living room carpets. But I really enjoy it. Through offering patience, understanding and space to grow and make mistakes, as well as acting as a role model, I can enable their development. Much as I think babies are lovely, I'd far rather deal with a snotty teenager than be changing nappies. It's just my style."

Sara Barratt, chair of fostering at the Fostering, Adoption and Kinship Care Team at the Tavistock Clinic, London, says many people choose to foster this age group because they've experienced tough times themselves or with their own children. "They feel they really understand how difficult it can be for these young people and unlike people who might go into something like social work for similar reasons, they want something that's home-based and more family oriented."

It's a role that's changed a lot in recent years, she says. "Mobile phones alone have made a big difference because contact can't be controlled in the same way. You might get a young person who, just as they start to settle and get a sense of self, gets a text from a really needy parent that sets them back."

Carers need to be good at communicating, have a lot of energy and able to be an advocate, she says. They need to be firm but not fixed, with good negotiating skills. They need a good supportive network, and must be robust and steady, with a good sense of humour. And whether they're on their own or in a partnership, and whether they've had children of their own or not, they need to be able to train. "As we gain a growing understanding of fostering this age group, there is a lot to learn."

Progressive fostering agencies and local authorities are putting foster carers at the centre of this process, says Andrea Warman, fostering development consultant at BAAF (British Association of Adoption and Fostering). "They're asking carers what areas they want training in – and offering them 24/7 support," she says.

Warman recalls a story about a youngster who'd been in foster care who sadly died later in life. "It was the people who'd shared a foster placement with her when they were teenagers that looked after her in her last days and made the funeral arrangements," she says. "When asked about it, they said, 'Why wouldn't we? She was our sister.' In fact, there had been no blood ties, but the bonds they had made with each other were stronger than with their birth families. That's the power of fostering in teenage years."