In your early 20s, and worried about changes in your 60s? Excellent. Here's a tip. Start saving for retirement NOW. And when you retire, you will be shorter, but you will have the $$ to buy a different bike/smaller frame. Most of my friends in their 60s or older, did not save for retirement, or saved very little. Life gets it the way, lots of reasons. But now the #1 regret I hear is not about losing height, its not saving for retirement. Saving early for retirement means you will be able to choose where you live, how you live, and what you ride (and a lot more). Plan for that rather than the 1 inch in height you will lose along the way.

#2 regret I hear is loss of mobility. Good tie in with riding. As you ride AND save, you will take care of the two biggies!