The person remembering your birthday may also remember who you were on earlier ones.

Long acquaintance is a form of custody. Someone who has marked the day for twenty years is holding a sequence: the year you were miserable, the year everything changed, the one where you turned up late and would not say why. They know the shape of your life in a way new friends structurally cannot, however close those friends become. That accumulated record is most of what old friendship is.

When the message arrives, treat it as more than a date remembered. Reply with something that acknowledges the history, and consider who you have been keeping that kind of record for.