I feel like snapping at my wife's step-mom when she keeps pointing out our age gap
Have you discussed this age gap before? Who believed up to now that you're the same age?
We didn't really discuss it before, but now it is all that my wife's step-mom can talk about when she's around us.
Your partner's step-mom is out of line. If she didn't speak up about this when you were dating her step-daughter, then she has no right to say anything now.
She doesn't care, she thinks my wife has low standards and is attracted to older men.
I am sorry she feels that way about you. Have you tried talking with her about how you feel? Sometimes people won't change and you can just avoid them.