Neither Tommen or Bran are 8 on the show. All the kid characters were aged up at the start, and they've obviously been growing as time is passing in real life and the show timeline.
I know they were all aged up - it'd have been pretty difficult to film most of the series otherwise, but just Tommen feels particularly young. Not just his appearance, but his lines too. I know it's intentional but it didn't stop it being uncomfortable.