First fictional boyfriend, huh? My interest in boys developed pretty early, as did my tendency to like older ones, so my first fictional boyfriend was probably Tommy Oliver (the original Green/White Ranger from Mighty Morphin Power Rangers) or Zack Morris.
So I was like seven at the time? Something like that? And I still maintain that these were good choices. If you didn't want to get on the Green Ranger or Zack Morris, I don't know what you why you were watching either of those shows.
(Tommy/Kimberly was one of my earliest OTPs, for the record. And I also remember being super into Zack/Jessie in that one episode where she was Snow White.)