Never write a horse off because of its age, you may be in for a bit of a shock
My old horse, a Part Bred Arab is still competing at the age of 22, and winning
The 'older' horse I used to ride when he was 12 until he was 16 (PTS a few weeks ago aged 34) used to dump me with great regularity and up until this last year could still pull the odd 'trick' to suprise you. To be honest I would have HATED to have ridden him as a youngster, lol.
What I would say is sometimes the danger with 'older' horses is that they have been there, seen it all, read the book, COULD write the book, and have learnt every damn trick in the book
They 'DO' have a sense of humour and will not be frightened to test your abilities, to see if you are worthy......
Don't be put off, boy will they still teach you a thing or two
I think 'older' horses actually 'make' you ride them and earn their respect.
Have fun, enjoy, and LEARN from the oldies.