Wouldn't mixing with a wide variety of ages from 18 to 118 be better?
You will work in the NHS or the firefighters and so on, so you will certainly engage with community members of all ages and from all wakes of life. For a year of mandatory service that everybody is meant to complete, 18 is arguably the best time to do it.
slavery
When you walk on a public road past a hospital do you think to yourself "they stole my money to build this"? You have rights and duties as a citizen. Your service benefits the community as a whole.
Also why is the stick to make people do it a fine (so that the rich will just pay if their kids don't want to do it)?