2015 - 1983 = 32 years
Why is it so unrealistic that Han & Leia and Luke & mystery woman (I won't say Mara Jade anymore since she could be negated by the ST) waited 5-7 years after the events of Return of the Jedi to have kids? If they did, the first born would be 27 years old at the start of the movie and would still very much be considered a 20-something. 20-something doesn't have to mean 20 or 21.
Leia would be having her first kid at 40 then (and Han 54). Sure medical science was better in the Star Wars universe, but you must admit having kids that late isn't typical. Especially for women.
Maybe Leia just froze some embryos in carbonite so she could focus on her career...
I think of it this way... Luke Skywalker is a hero - like Lincoln. Did amazing things. Made history. Saved humanity.
Nobody gives one damn about Lincoln's kids.
I think you are getting way to hung up on the actors' ages instead of the in universe age. If the ST is set 30 years after ROTJ, then Luke and Leia will be ~52 and Han will be ~62. If you follow the EU for the birth of the Solo kids, they would be in their mid 20's 33 ABY since the twins were born during the Thrawn novels which were five years after ROTJ. Ben Skywalker was born a lot later than that. I am assuming the EU will be essentially wiped out by the ST, so I doubt it will be the same set of kids, but it certainly isn't unrealistic for the second generation to be around the same age Luke, Leia and Han were in the OT if the ST is set approximately 30 years after ROTJ to coincide with the actual passage of time in the real world.