I never did purees, so I don't know when you are "supposed" to stop but I would just do what you feel comfortable with. Don't let the random cashier bother you. I had one say to my LO "Tell your mommy that she needs to buy you warmer clothes, you're going to freeze!" Because I bought a long sleeve shirt and leggings for her in December. :roll:
We stopped purees when DS stopped expressing interest in them, around 14 months. He preferred being able to feed himself at that point. I'm sure we would have done it longer if he still liked them!