I saw a sign at Knott's Berry Farm when I was a teenager that read, "Don't regret growing older. There are few people who have that privilege." I have always remembered it and kept it as part of my value system. It helps me enjoy my time on the planet with very few regrets and also gives me a confident attitude about the future.
If you don't like your age, that negativity will trickle down into other areas of your life and chip away at your self-esteem. Look, you are who you are, and you are the age you are, period. Accepting who you are, along with how old you are, is the only way to have a fulfilled life. If you mourn for your youth, then you won't get to experience the calm that can come with experience or enjoy the wisdom that accompanies age.
James Taylor once sang, "The secret of life is enjoying the passage of time," and I couldn't agree more. I'm sure there's more than one secret, but this one is a goody. If you can't love each day and squeeze as much out of it as you are able, then you will go to sleep with regrets and wake up with anxiety. Trusting that you are in the right place, at the right age, with the right people, will give you more joy and satisfaction than a roomful of self-help CDs.