TBH you just have to live with it. One day you may be grateful if you still look younger than your real age, otherwise you'll look back wistfully on the time you looked so young.
I always looked very young. When I was 22 and getting a new pair of glasses at the optician, she asked me what school I was at and what gcse's I was studying. I just laughed it off - at least at 30 I still look young for my age.
There IS no way of asserting it without coming across as self-important/insecure or, ironically, as emotionally immature through your need to push the age issue.
... and the ones that won't