For example, one partner earning substantially more money than the other is also an unbalanced power dynamic.
Key differences here.
Age is easily calculable and society already legislates around age, so there is a discourse to be had about what the exact "rules" should be, whereas other "power dynamics" are more complex and nuance, it wouldn't really be possible or desirable to have the police monitoring people's financial status to make sure they weren't sleeping with people significantly poorer than them.
Also, with age and sex, everyone (besides AnCaps) seems to recognize that there is definitely such a thing as a "child", as in someone too young that adults shouldn't be having sex with them (and for very young children just shouldn't be having sex at all), age gap discourse just seems to be an extension of the general cultural debate about where exactly that line is drawn.
Probably the smartest move.