Seriously? There's an age limit for boys to go into a girls rest room? I've never heard of that, and if I did, I'd totally ignore it. You do what's best for your son and don't worry about what anyone else says. My son walks right into the woman's restrooms (at age 8) and no one has ever said one word to me. And if they did, then I'd have a few things to say for them.

I agree with everyone. I let my son go with me until he was 9. No one said a word to me. Thank goodness most places have a "family" restroom because my son is 14, soon to be 15. As much as I hate it, I have stood outside men's room. I have even told my son to go into the men's room and sing LOUD! so that I can hear him. We all do what we have to do.

If a family friendly bathroom isn't available I just take my son in with me. If I get ugly looks or questioned I simple tell them why.

My son is almost 10 years old and I still take him in the womens bathroom with me. I've never had anyone say anything to me but if they did, I would be happy to let them know why my son is with me : )

Honestly, I would try not to stress or worry over this. First, amazing that your son is potty trained!!!
And second don't worry if someone judges you. Your son is your priority and if someone has something to say, well then they are the one with the problem. My son is also 5 and always come with me into public restrooms.I wouldn't have it any other way.
You do what you need to for the sake of your child and to protect him.