Hi Jen! It sounds like you are doing a great job!
I am currently nursing my 3rd child & I let them nurse until they weaned themselves as well. Both of my boys weaned themselves at 10 and 10 1/2mo. I fully planned on nursing them until at least a year old but they had other plans.
It is completely up to you, but i would think that breastfeeding until at least a year old would be best. Just like if your child was on formula and taking a bottle. But i think that it is great that some moms nurse much longer. I think that it is so beneficial to the child that the longer, the better! I do have to agree with Stacey though. 5 years old is a little much! I can't even imagine my 5 years old nursing! As in your MIL saying that your baby's mouth will get deformed...I am sorry but i just had to laugh! I have never heard of such a thing! Babies who drink from bottles, yes, but i have never seen a breastfed baby have his front teeth pulled from them decaying or thier teeth sticking out. These are all common to bottlefed babies. Who knows though. Maybe Ann, our breastfeeding expert has an idea on this one!
Keep up the good work. Your baby is still young and as long as you don't mind continueing to breastfeed then i say "you go girl!"