My DS is 10 years old, and he would be sooo embarrased if I actually suggested that he come into a Women's bathroom with me. He's been using the Men's room for numerous years now--I just wait outside the door for him, if necessary.
Personally, I think 10 years is a bit old for a typical boy to still be using the Ladies' room (unless they have special needs, there is a reason for them to require adult assistance/close supervision, and no other reasonable option is available--there are always exceptional situations). I am all for taking younger children into a restroom with you (regardless of gender)... but there comes a point when children should be allowed to begin to use the appropriate facilities.
At Disney, and a growing number of other locations too, they do now have 'family/companion' restrooms for those who want or need that option. And if you are really nervous about allowing your 'tween-age' child to use the gender appropriate restroom on their own.. consider the options of giving them a cell phone or radio (which they can use to contact you) &/or teach them to yell and run away if anyone approaches them in an obviously inappropriate manner. If it buys you peace of mind, any slight inconvenience may be worth it.