"It is never too late to be what you might have been."
- George Eliot
People don't say "you're only young once" because they like the sound of their own voice. It's a concrete fact. Yet, as far as I'm concerned, you are also as young as you feel - believe it or not these aren't mutually exclusive.
The former interjection is usually an effort to prevent you from wasting your time in this short life we all live, (by their judgement). This is not to say, however, that you cannot lead a fulfilling life over many years, or that you must try everything before you're 25.
Expecting or yearning never to get old isn't the answer, but celebrating every moment of your felt youth definitely is.
Time is both infinite and spectacularly, fleetingly precious. There is an abundance and yet not enough. You are both young and old, free and tied down. You are every emotion you can imagine and none of them all at once. We are the embodiment of time, and we are forever becoming, but don't be taken up with the tick-tock of safety - take a risk. Because you can. Whenever you choose.
Don't write a to-do list today. Do that thing you haven't had time for.
Read up on mindfulness, the importance of patience, meaningfulness, emotional intelligence and the power of introverts