I'm lucky that I look quite a fair bit younger than my actual age. I'm early 40s but frequently get mistaken as being in my 20s or early 30s. I've always looked younger than I am, even when I was younger. I was even asked for ID last weekend when buying some craft scissors, and the girl working in the shop was shocked when I told her my date of birth. She said I was 'one of those people that aged well' and she was amazed I was actually 10 years older than her. Someone else (a guy) recently said he liked my style (referring to my hair) and thought I looked about 30.
So in that respect, I can still pull off a C contour hairline, a reasonable amount of density and no grey hair. I have a longer hair system also (around 10 to 12 inches). Maybe I won't be able to do this forever, but it works fine for me right now. The hair system adds to making me look younger too. As I get older, I might get tired of longer hair and go for a shorter style, reduce the density a bit, add more recession to the hairline, maybe even some grey, who knows? But right now I enjoy longer hair and youthful look, so no need to change anything. I count my blessings.
If you're someone who looks their age, or looks older than their age, especially if you're late 30s or older, you may want to consider making your hair density, hairline contour and level of grey 'age appropriate'. But on the other hand, even if you are getting older, you'd be surprised how having 'younger looking hair' can actually make you look younger too. The way I see it, I didn't start wearing hair systems to look 'age appropriate', I bought them to give me a bit more of a youthful appearance (even with me looking younger already). If I wanted grey hair, a receding hairline or low density hair, I wouldn't wear a hair system in the first place. I wear them so I don't have to have those things.
But that's me. Some guys NEED to go with a more 'age appropriate look' as not everyone can pull off younger hair types. You just have to make that judgement for yourself and do what works for you.
I don't think you necessarily have to go for a less 'cool' hairstyle just because you're older. That's simply a personal choice. Whatever you do, do YOU and rock it with confidence!