Do I base age off of babies due date still if born early?

Nicole • Full-time momma, catholic. I drink a lot of coffee. 💙

If my baby was born 3 weeks earlier than his due date will he still be considered one month old at four weeks, this week?? Or do we always go by the babies due date as their actual age? I’m just curious because it doesn’t feel like to me he is one month old already. I mean he was born early so if his actual due date was only last week then technically he’s only one week old right? How does that work