Interesting question. I'm sure you'll get different responses.
For me, "mature" indicates depth and complexity. Maybe layers.
I'm not even sure how to characterize the opposite except to say that it would be fresh, lively, more focused on one or two elements. "Young" doesn't quite do it justice.
Some soliflores might be called young. Thierry Mugler cologne seems young and cheerful to me.
Some scents with evolving complex stages might be called mature. A scent with a wistful, contemplative, even melancholy air might be called mature. Darker and earthier notes. Or very austere notes, like a dry incense or woody scent.
Each are great, each have an appeal.