I'm 5'8 and ride a KX250. I don't know the exact seat height, but they are all roughly the same. I have seen some people stand on blocks or even their trailer to start them. I just throw a leg over, turn the lever out, and lift my leg. I have had to use my arm to pick my leg up that high before. If you do find the CR to be to high, you can get lowering links for it, or cut down the seat foam. Remember, Carmichael is only like 5'6". Ask the dealer to let you sit on the bike before you buy it, and see just how hard it is to get you leg over and onto the start lever. Remember that it will only get easier. And don't be afraid to try the other makes if the Honda sits too tall. All of the Japanese bikes are really good. Personally, I do not like Honda or thumpers, but go for it.
And yes, there are classes for people your age, my local series has +30, +35, and I think it even has a +40. Also, in may I plan to enter my first race in a 30-39 yr old class.
Good luck, and happy trails.