How old is everyone?

I just can’t figure it out. Since Spirit was 18 when Maka was born I’d estimate he and Stein to be around 35. I’m trying to figure the ages of pretty much everyone. I can’t figure if Marie is older than them or the same age. I think the main characters are around 14-15, but if you consider that the new students in NOT are 14 I lean more towards 15. Ideas?