I don't think it's letting yourself go, it's you being you. I'm in my 20's and I've been finding greys for years, not many but they have bee discovered. I'm not ready to accept that, haha, but if I did decide to let myself go grey (if it were coming to that), it's because I' would be willing to accept that this is me, it's my hair, and I have better things to spend money on. Plus the damage it can cause to your hair.

Do what you feel happiest doing, and what makes you feel more confident about yourself.