30 is the new 20 and 40 is the new 30 blah blah blah.
I think being "old" has more to do with you how feel rather than how many candles are on your birthday cake. As far as feeling old in relation to the reflection in the mirror, I started to notice fine lines on my face, so I started using moisturizer.. problem solved... when I started getting bothered by gray hairs, I started getting my hair colored and where this sort of maintenance can get spendy, I think personally my vanity is closely tied to my perception of my own age. I feel I look younger, I feel I act younger, so generally the perception of others is that I am younger.
Funny thing, I went to the liquor store a couple of weeks ago with my baby brother who is 10 years younger than me. We were in line, he went first, bought a case of beer without question, I get up to the counter with a couple bottles of wine (I was making spaghetti, the wine is key to my sauce) and I get carded. I had to laugh at that.
As far as living forever... who knows how many of us will die of old age and how many will have an accident or whatever. Thinking about mortality in that sense is a great waste of time because you just don't know. Better to live in the now and deal with what you see in the mirror today than worry too much about what you may or may not see tomorrow.