Sting does have a son named Jake, but Jake was born in 1985. This makes him seven years older than Ben. However, the show is actually referring to Sting's real-life son, Giacomo ('Jack' in Italian), NOT his older son Jake. Giacomo would be Ben's age.
We have seen Monica wear the same boots during the previous season.
An over-arching theme of this episode is how expensive Monica's new boots are. Given her perfectionist nature and tendency for OCD, Monica would have thoroughly tested the boots before she forked over a significant amount of money for them. The fact that they are too painful to wear - another major theme of this episode - would have become clear and she wouldn't have gone through with the purchase.
When Ross tells Phoebe that Ben is 7, she expresses surprise, saying he's small for his age. In actual fact, Cole Sprouse was about 9 at the time, if anything he would have been big for his "age".