DW and I are 23 and 21...people are always for some reason assume we're brother & sister instead of married. Whenever we tell them that we're married they're like "WHAT!!?!? You both so young.." So then we're both just like well we've graduated college with Bachelor's degrees, live 1,000+ miles from our parents, and take care of ourselves....but thanks. It gets sooooo annoying.
Almost 2 years ago DW was 19 and buying stuff for our wedding and my MIL walked away to look at some stuff and the lady at the cash register asked DW who was going to sign for the purchase and when DW said she was the lady was like "how old are you?!?"
Two or three days later we're at Kohl's buying pants or something and the lady at the register questions DW's purchasing abilities by saying "you're just so little"...I stood there

waiting for DW to climb over the register and strangle the lady with whatever we were buying. DW is barely 5 foot so it's not like she's handicapped or anything....we sooooooo should have just been like thanks not let me talk to your manager. lol
Usually after someone comments about how young I am I start growing my facial hair out...lol