I'm 30 now and have dealt with looking like I'm in my early 20's for years. I used to wear a stone face and hide the light in my face, because a genuine laugh or smile instantly makes me look "too" young. Doing this made me look older and helped people take me more seriously. But I suggest not doing this strategy, because I found that the practice actually ends up negatively affecting your spirit over time.

Also, there's a lot of people who just be jealous and are projecting their own insecurities onto you. The negative people in your age bracket are jealous that they will not be looking as young as you in their 30s/40s/onwards. The older people are jealous that their own children aren't financially stable enough to have kids at a younger age. Etc.

I maybe sound resentful. Sometimes the excuses I listed above are applicable, sometimes they're my own projections. If you have the same tendencies as me, then I offer this advice: try to do what you helps you retain the most of your joy.