J's Age

So I'm watching the first movie and K says in the end after they are sitting in bug goo that J should have been there for thezerellian migration of 1968... Then proceeds to say that he probably wasn't even born yet.... Hmmmm

I think K may have had J's birthday, plus a few other life details changed to protect J from the truth. We saw K neuralise J as a kid, he probably sent for the rest of J's family (presuming he had any) to be neuralised aswell so that they didn't know what happened to the father other than he was "a hero".
This could also explain K saying he should've been there for the migration in 1968. He knew J was actually alive at this point but covered himself by saying "you probably weren't even born then..."

In all honesty I think it was an oversight by the film makers, as I feel J would've been a little taken aback once he realised he was a good 5 years older than he thought. I'll explain this away though by saying he probably was unhappy about the age gap but he was having a nice moment with his partner and didn't want to spoil it. He probably mentioned it days later.