An eight-year-old can have friendships lasting longer than three years. That doesn't mean he/she is ready for an intimate relationship with someone twenty years older...
An 8 year-old doesn't have long term friendships with adults.
Look, I'm simply pointing out that I'm loathe to declare that someone's relationship is less real than someone else's. I can accept that that people dislike C/7 because they found it rushed, that they feel the characters have nothing in common, that they feel the characters are too alike in personality, that they saw no chemistry between the actors--and hey, even to argue that a 20 year difference in age is "icky."
But to argue that Seven is a mere child incapable of adult relationships is simply ignoring how the character behaved and grew over the prior four years.