i know an "irresponsible" woman who had a child when she was a teenager. she was 40 when her child was in college and ready to move on in her life. if she lives until 80, she'll share 60 years of experiences with her child on average.
if some professor decides to wait until they have tenure (say 40) before they have kids, they're likely to have 40 years of overlap. they'll be dealing with an unruly adolescent when they are 55.
if you have a kid at 50 (some people do!) then you have 30 years of overlap and you're dealing with an unruly teenager at 65!
(Oddly, have kids at 30, like I did and my dad did, and you're in your 40s when your kids are in their teens. The 40s seem to be a time that parallels adolesence and a number of ways... Stan Lee and Mike Ditko were in their 40's when they were imagining the adventures of teen Peter Parker...)
so having kids late in life means that parents have less experience with their children and children get less out of their parents.
(note that smaller nuclear families have an almost genocidal effect on extended families... if you have N siblings on average, you have N^2 cousins!)
for years i've believed that late childbearing has an effect similar to an increase in background radiation, this study confirms that and just adds to the case