I know many smoking-hot middle-aged people who are emotional teenagers.
You can date someone as young as half your age plus seven years? It's no wonder that so many women feel that men their own age are too immature.
The “half your age plus seven” rule of thumb probably helps you avoid being a creep, but is it accurate?
Much to my surprise, when overlaying a line representing the rule of thumb on Kenrick and Keefe’s results (see red line in the figure below), you can see it closely matches the low end of the range (i.e., youngest preferred) for all but the oldest men.
"Acceptable" is such a loose term, so it's hard to say what is "acceptable" for any given person.
After all, everyone is a different individual and their situations and maturity will vary greatly.