On adding bike riding and leg extensions to the program:
The program in [Starting Strength] is not designed to be supplemented with anything. Neither of you -- old guy or young kid -- are in a position to recover optimally, as would be a 21 year-old genetic freak.
Just stick with the program and let it work for you unsullied by the trappings of muscle magazine silliness or a desire to gild the lily.
If you want to ride your bikes, don't be gone too long.
In fact, be back before supper.