We don't know Castle's birthday yr but I assumed he's 43 in the show currently cause he said this happened 30 yrs ago and 1983 is 32 yrs ago and Nathan is 71' so Idk where 69 came from haha I think him being 11 in 83' finally shows us his age unlike Beckett's who we found out in S4 her age when they were wheeling her into the hospital