Welcome to the forum
you're never too old if you have experience on a mountain bike then a 26" would probably feel more comfortable.. also the length is more appropriate for your length.
The only difference your age makes is that you may get tired easier and you're more aware of the dangers, but there is no reason why you can't do it
Go for it fella, have fun riding
As for bike you're best off going second hand, try looking in the for sale section on here, you'll find you can get a decent spec fairly cheap with little damage from friendly people