I purchased a book about ASL before my boy was born. I started reading it when he was a month old and it encouraged me to sign to get into the habit. So I began signing milk and diaper at 2 months old because he was perpetually hungry and hated being wet.
At 2.5 months old HE SIGNED MILK AND DIAPER! We couldn’t believe our eyes! They didn’t look exactly like the signs but his body language to tell us he was wet or hungry changed to a downward tap at his waist and a closed fist instead of what we called “swimming for the milk river.”
He did’t pick up any additional signs for a long time. Then at about 6 months he started signing light and fan. Then nothing for a while and at 8 months dog. ASL took off.
Keep trying and it can work! At 17 months he knows well over 100 signs and can approximate 90 of them himself.