They actually did a study a few years back where they got a bunch of guys to wear replica or actual 17th century horseman's armor and then told them to get on a horse. None of them had problems (They were all trained riders).

What they did discover was that some pieces of tournament armor would make it nearly impossible to mount, and it seems likely that jousters would mount and then the squire would put the last pieces of armor on.

Oh, and don't get me started on what modern people think "holding the stirrup" for a knight to mount meant...