Why do people ume when they see an older woman with a baby that its her grandchild??

HI, I am 40 yrs old but really dont think i look my age at all, my problem is i had my son when i was almost 39 i wasnt supposed to be able to have anymore..but boom 16 years after my last baby i was told i was pregnant lol i was totally shocked but to get on with it....when i am out shopping pple will say ohhhhh what an adorable grandbaby u have hmmmmmm i just smile and say well not my grandbaby but my son and thank u. should this bother me...i try not to let it...but after a while it gets tiresome...and i really dont look like a grandma lol guess i just need to let off steam thanks for reading and listening God Bless u all