That was well below 18 for most of the time I have used the Internet. People born on that day were toddlers when I started to seriously use the Internet.
I could nowadays enter my real DOB and get through all checks but I usually still pick something in the 1970s or 1980s.
If the default date is old enough to get past the prompt, I use that one. If it isn't, I pick a random year that is. I don't have to lie unless I want a senior discount or something, but I just don't want to share my birthdate with any random site or service.