I am 30 and have had a beard since I was 19. My freshman year of college I was too lazy to shave and just let it grow. I kept it trimmed, but it did make me look older, which was a good thing when I started a professional career later that year and was working with people in their 30s, 40s, and 50s. It helped me and my words be more readily accepted. Someone in their 40s doesn't want to receive professional business consulting from a young whipper snapper. But someone who looks to be in their late 20s to early 30s? Not so bad.
But now I have to keep it; my wife will not let me shave it (she says I look like I'm 13 when I do). I used to keep it trimmed pretty short, but with the current popularity of longer beards, I let it grow out some, but not to Duck Dynasty length, keep it looking decent, and trim it down short every few months.