Say you've just turned 26. You realize that most pop stars and actors are younger than you. People born in the 2000s are teenagers now. And, you realize you have adult things to worry about, like keeping a job. You're older now...
But are you actually old? No. No you are not.
Seriously, just stop saying it.
Watch the CollegeHumor sketch above to see why you shouldn't be calling yourself "old" unless you're already worrying about hip replacements or retirement.
And even then, you could say it's a grey area.