My father is 70, my mother is 5 years younger, so 65 (though there’s a brief period of time when she’s 4 years younger, that’s not now).
My brothers and one of my SiL’s, though, I generally have to do a chunk of mental gymnastics to get to, even though it should be easy.
So, when my brother got married it was the last summer I lived in a specific town, so 1999. He was 30 when he got married. That was 12 years ago, so he’s 42 right now, will be 43 soon. His wife is 8 years younger than him, she’s 35. My other older brother is 2 years younger than our oldest brother - except now, when there’s only one year, since he just had his birthday. He’s 41. Younger brother is 30: I know what year he was born, so he’s the easiest one.
Up til 6 years ago I didn’t know exactly- my Mum never told me her age or birthdate -she also used to claim 21, until my brother hit 21, at which point we told her to quit it- then she finally decided to tell us when she was due to hit 50, at which point she got terribly childishly excited, planned a massive party, and started inviting people over a year in advance.
I always knew her birthday though, as it was the normal first day back at school after the summer holidays, and we always made a big deal out of it.
My Dad I had a vague idea about as well until his 60th- he too had a pretty big party, but mainly because he’d always wanted his own hog roast and a barrel of beer with his name on. I know his birthday now, after he changed the burglar alarm code at work to the date, after we all forgot it once. Mum always reminded me before that, but that year she forgot too.
My husband’s mom is almost exactly a month older than my mom, despite a 5 year gap between my husband and I (I’m older). It makes it really easy for me to remember how old my MIL is. I have only a vague sense of how old my FIL is. I know he is older than she is.
My mom did this for both her 50th and her 60th (which was just at the end of last month). Huge parties in rented venues with catering, invited people over a year in advance, made sure it was the most important event of everyone’s calendar for the months leading up to it, etc. (For her 60th she hired a Scottish bagpipe band with the drummers and everything, an Irish stepdancing troupe, a Scottish highland dance troup, and a Greek traditional dance troup to perform at various points during the evening to celebrate the heritage of both her birth family (who she found only a few years ago) and her adoptive family. It was a big production.)