I'm 16, I recently found out I'm dating a FTM transsexual who is almost 26?

This is a tough one. I would have to say it would probably be best to call it off. With that large of an age gap there is a good chance it wouldn't work out in the long run anyway and, you are kind of putting him at risk since you're underage. The mistake is totally understandable as it takes a while for trans guys to look their age. I doubt this is helpful but I do hope you guys find an arrangement that works for you both and is safe for him.