'For 15 years' is technically the correct way to express that I last played sports was 15 years ago. That being said, you will hear native English speakers say 'in 15 years' in casual conversations. Both statements have the same meaning.

Here, “for 15 years” shows the duration of the situation continuing over time. There is more emphasis on the fact that I haven’t played sports at any time during these 15 years.

They both have the same meaning - the last time I played sports was 15 years ago. 'I haven't played sports for 15 years' has an additional interpretation. It could mean that the person has played sports continuously for some time but that time is not 15 years. It is 10 years for example. But this would be very rare, and would be clear from the context. Without context, it has the same meaning as the other phrase.