Not really

Since the Amish believe in adult baptism, they do not engage their young in the whole community life. As 12xU mentioned, Rumspringa works fine for them to confront the youth with the outside world so they can choose, informedly, whether to leave or to stay - and then they get their adult baptism if they choose to stay. Currently this happens at 18. I don't know really but I can believe it may have happened earlier in previous times when 12 year olds from the outside were already de facto adult. It certainly isn't the case now, so no, all Amish marriages, being post-baptism, are between adults.