Dating is not illegal. Sexual relations can be. If you are not 4 years older it is not statutory rape. But there is coercion that could be alleged now or later if she gets upset.

Is it weird for a 16 year old to date a 15 year old?

The answer is No and No. A 15 year old cannot legally have sex with ANYONE. The 16 year old could. This means that the 16 year old can be charged with statutory rape and sentenced as a sex offender requiring among other things