My 17 (!) year old cousin is dating a guy who went to kindergarden with me, so my age. I just remembered that. And she has been dating him since she was 15 and he was 20.
Age is irrelevant. It's all about maturity.
It should be about maturity. However it's quite naive to completely dismiss age as a factor.
Anyway, there are always anomalies, instances that break the norm. This is just one of them.