My DC are now in their twenties. We've lived in WG for almost 30 years, and it's been a great place to bring up children.
WG is on a train line between Welwyn Garden City and King’s Cross/Moorgate. Most DC around here go to school in Brookmans Park or Potters Bar (Mount Grace/Gracewood Academy or Dame Alice Owen), so that works out well for independently meeting up with friends, or going into London. However, transport to St Albans isn't so great. I haven't known any children go to school in St Albans, so can't comment on that.
My DC loved NMYP, which was mainly children from WG and Brookmans Park schools (WG and BP make up the parish of North Mymms). I don't have any recent experience, but back in the day, they went every week, and did all sorts of fantastic indoor and outdoor activities. However, they had a lot of school friends who also went. Your DC might find it more difficult if they were shy and didn't know anyone. Also, by 14 or 15, my DC tended to do other things.
Have you posted on the local Facebook groups? People there could give you more up-to-date information.
WG www.facebook.com/groups/208030496542251
BP www.facebook.com/groups/500965523343858
All in all, it's a lovely area, with lovely countryside, nice walks, etc although (like everywhere near London) there are plans for new houses.