The comment 'You are too young to be a professor.' doesn't sound like a compliment. The implication is that he/she isn't experienced enough for the role. 'You look too young to be a professor', if aimed at a woman, is probably meant to be a compliment.

Actually, it sounds offensive to me. Why couldn't young people be professors? If they are smart enough and developed themselves so much, what's wrong with being a professor at whatever age?

I think it's remark or fact , it don't seems a compliment.

I really, really don't know. We'd need a lot more context than one single remark.