I don't see where the 5 Robins mean Bruce has the be in his 40s, if he started out as Batman in his very early 20s, and each one only lasted a year or two
Yeah, but that's exactly the problem -- that you have to compress the Robins' careers into such an absurdly short time frame. Especially Dick, who started out as a preteen, literally grew up as Robin, struck out on his own as a Titan, and then became Nightwing as an adult and spent years building an independent reputation. It serves his character poorly to cut his Robin career down to just a couple of years.
I once worked out a chronology for the DC Animated Universe, assuming it took place roughly in real time. I had Bruce become Batman at age 22 (in 1984), taking in the 9-year-old Dick Grayson as his ward later that same year. Most of B:TAS is in Bruce's early 30s, and he's 34 when Dick graduates college at 21 and gives up the Robin identity. Batman is 36 when he begins training Tim Drake (there's no Jason Todd in B:TAS, though its Tim is basically Jason with his name changed, just as its Dick was an amalgam of Dick and Tim). He's 38-39 when he co-founds the Justice League, and he's 41 when the flashbacks in
Batman Beyond: Return of the Joker take place (and Tim would have to be 17 despite being depicted as early teens). Bruce is 43 at the end of
Justice League Unlimited, and 59 when he retires in the prologue to
Batman Beyond, though I've always found that highly implausible, though I suppose the Watchtower's advanced medical technology would've helped. He's 79-81 in
Batman Beyond.
So the DCAU Batman only had two Robins in the course of 18-19 years, plus one Batgirl, of course. It's pretty hard to imagine cramming four or five Robins' backstories into just five years as The New 52 tried to do.