The family in my test hood were direct from CAS, and they're affected in the same way - new teens are too high up on the fertility curve, and new adults are past the peak. It doesn't seem to make a difference whether I've aged up and down or whether they're brand-spanking-new.

And yes, despite the low odds, my sims ARE autonomously woohoo-ing, when they find the time to do so! I did notice a rather high number of hours on one sim's "clock" to what I might expect, but it worked fine once the number got close to 0.