Wow - sorry you are dealing with this! It's frustrating. I'm soon to be 40 and my husband is soon to be 60. I get mistaken for his daughter all the time. I am am short and skinny, and can be mistaken for early 20's (late teens on a good day) if I'm not wearing make up or business casual clothes. We have heard some really vile comments so I can only image what has been said to you. Usually when someone says something disgusting to me (comments I get usually involve incest on some level eyeroll) I look at them seriously, wait for them to finish laughing at their ridiculous joke and ask "was that an attempt at humor?" I don't change my expression. Then I just move on to the next the next thing I was going to say. More often than not people drop it, though some have continued in which then makes me say something rude.
Regardless of appearance people should respect your relationship, particularly as this is your son. If they continue or it's something truly vile turn it around on them. Odds are they probably haven't maintained themselves and don't look as young as you. I have learned that mentioning someone's wrinkles, poor skin, or weight gain gets the heat off my appearance. Feel free to mentioned taboo topics in return (weight, age, etc) - and close with they look like they're just waiting to die...how sad.
I'm sure your son doesn't want to hear implications that your dating him any more than my husband having me compared to sex with his daughter (he has five adult daughters). I don't understand why people are so gross and rude, and feel comfortable expressing those thoughts to complete strangers. I have a pretty dark sense of humour and can see the humor in taboo subjects but these types of comments really aren't funny, though I will admit there have been one or two that we still laugh about. I had one woman who kept offering me bubble gum at a convenience store once, she truly didn't get it. We also had a bank teller offer me candy while my husband made a deposit saying how well behaved I was (I don't look that young).