The majority of the trainers I see at my gym are 35+. Though, I don't know how early they started. I know one has been competing since at least as early as '83. She's also appears to be in the better shape than any of the other trainers. There's actually only one under 30 trainer who is in top physical condition.
Personally, I trust age and experience more than I do beauty, youth and boundless energy - in most everything in life except for maybe, "How do I send a text message without it taking me 20 minutes to type 10 words?" Then again, I'm nearing senility.
If I were to hire a trainer it would certainly be someone nearer my age who I feel could relate to me on a personal level. In my experience, fitness has required lifestyle changes. In order for you to help me you're going to be able to "connect" with me in areas of life beyond the gym. For me personally, that rarely happens with the under 30 crowd. That's just me though. I'm sure there are other old people who think old = no clue about fitness or maybe they think youth is contagious. If only.
FWIW, from the small sample I've observed it seems the young trainers seem to have a younger client base. Vice versa for the old folks.