Is ok for a 13 year old girl to date a 16 year old guy?

It's not a healthy relationship. A 26-year-old guy is showing an amazing lack of emotional maturity going after someone so much younger. It means he hasn't really grown up like the people his age. The girl is better off dating guys closer to her age.

Later, the age difference won't matter nearly so much. Once you are 21, for example, dating someone 30 is still odd but not a waiting disaster.