Even if we say I agree with this, why even ask for a specific year? Separate into child and adult, and let the super user make that change when asked.
Different countries (actually different regions within said countries) have different laws related to what "kids" can and can't see and what age defines a "kid". How much that matters is up to you. But it provides an automated check that ALSO avoids having to say "Hey mom? I just turned 18 and for no reason whatsoever it would be great if you could switch my account to an adult. Also make sure to knock and don't look too closely at my laundry basket ever again".
Websites and applications should never be allowed to know any PII without explicit consent.
And what do you think you are providing every time you tick "Yes, I am 18 years or older" or "Yes, I was born in 1920 or whatever the first option is now"?