I have no age restriction on either side of the scale. Everyone gets one chance to prove themselves! I've found that age doesn't always predict behaviour.
Certainly maturity is a welcome factor, but here are 4 things I really like in a client:
1) Respects my privacy!
2) Communicates appropriately (expresses appointment request clearly and preferable without going back in forth a million times!)
3) Show up on time or with a note if he's going to be late
4) Is respectful during our time together.
Pretty simple! I'm sure I'm not the only one who thinks this way.. but I don't really care about looks, ethnicity or age!