I don't think I know any 21 year olds who
a. Have their own home or
b. Have the maturity to parent children with often complex needs.
I think it's a bit strange to be targeting young adults in their twenties to be honest. It would be better to focus on more mature adults in their 30s and 40s and older.
I do foster, and I was fairly young, early 30s, when I started. I had a lot of experience working with children but I hadn't had any children of my own. Looking back I think I probably was very naive and had unrealistic expectations of what parenting these children would be like. I mean, we coped, and 9 years later the children are still with us.
My LA liked foster carers to have older children than those they foster, but I have heard they have recently relaxed this rule, which I think is a massive mistake. I now have a child of my own who is several years younger than my foster children and it's not the best fit. It would be much better for all concerned if she was the older one, but she was a bit of a surprise and that's just the way our life panned out.
In my opinion it is much better for people to have their own children and let them grow up to at least mid childhood before introducing a much younger foster child into the mix. Plus as others have said, if there are any allegations, your own birth children are at risk of being removed as well as the foster carers. Fortunately it has never happened to us but I have heard from several other carers that if allegations are made against carers, the foster children are whipped out of your house immediately, then investigations take place which can take months, and then even if you're found innocent (which happens in the majority of cases) the foster children don't come back to you and you may never see them again. Which I suppose you might not want to if the allegations come from the children but in lots of cases they come from birth family or as a result of the children having an accident and breaking a bone, for example. Depending on the nature of the allegations your birth children might be removed as well and could go to family members if appropriate, or they'd end up in the care system as well. And you might only be able to see them once a week in a contact centre. I understand why people with their own young children might not want to take the risk of that potentially happening.
Fostering is not a job. It's your whole life. You don't just house the children, you make your house their home. You have to get involved with their school, their hobbies, their birth family. Young adults who are enjoying going out every weekend are not going to want to swap lazy Sundays after a boozy Saturday night for Sundays spent taking a 6 year old to a whole class party in a soft play centre.
Personally I think it's much more sensible for people to party, travel, have a career, raise a family all before they foster, if they want to, later in their lives.