Hey Bob
I noticed that change too from the raging wood teenage years and entering early 20s (I am 23 now). Seems like there shouldnt be a difference but there was. Its nothing to be concerned about too much.
I could be anything: diet, health, etc. One thing it might be also is stress. At 21 your priorities and goals are different than at 17. At 17, my life revolved around getting head and getting high, and oh yeah, graduate from high school. Stress was pretty low. Now I worry about job security, getting married, rent, car insurance etc. Ass and titties have moved a few peg down on my daily thought process haha. But I make time for sexuality with my girlfriend and hell, even make time for me. It does me (and my erections) a world of good.
This is one of the reasons I do PE exercises. I am quite happy with my size now (I realized my desire for a bigger penis was due to insecurity in other parts of my life) and just focus on strength.