That's interesting. I'm an adult with an ASD and have problems remembering age too. I thought it was just me.

I forget how old I am, but I know my birth date and just do the math in my head to figure it out. I have no idea how old my wife, kids, parents, or siblings are. I do remember what year they're born though and just do the math in my head.

Your son is probably too young for doing the math, but maybe try something else besides just telling him his age. Have him show you with his fingers. I remember things much better if I can visualize them my head.

My son's teacher made him a laminated "All About Jake" book that helped him learn fun/important things. He learned his birthday/how old he was, his address, my first name, my husband's first name and my cell phone number. We were terrified he wouldn't be able to tell anyone any information if he ever ended up away from us or his teachers. We even wrote the book into his IEP to use it as a goal for him to learn the info. There was a page showing him swimming and playing on the computer; those activities are what he answers when people ask him "what do you like to do?"

Can he answer the question 30 seconds later rather than 5 mins? If not I would test speech and receptive language? Perhaps he doesn't understand age and what is 6 in context of the age or the question itself.

I'd suggest writing a social story that you read to him daily until he remembers. One girl I worked with thought people were asking her, "How are you?" when they were asking, "How old are you?" I wrote a social story for her with pictures and her parents read it to her daily. She soon could answer both questions correctly. Ask your child's Speech Pathologist (if he has one) to make one if you need help. If your child can recognize numbers, you can post his age in his room and on the fridge as well. Then you can review it often with him until he remembers. He'll feel successful, because he'll have the number to use as a visual cue until he remembers.

My son will always say how many are you vs how old are you. maybe your son doesn't understand the terminology of the question your asking.