My $0.02 having no idea what you look like or your age or weight... I'm ~50 also, and I don't think my BMR has changed since my 20's. People's activity levels change, though, and in your 20's while you were out moving shit, doing sports with your friends, going to the gym even when you were tired because who the fuck was ever tired in their 20's... your natural activity level was probably off the charts in your 20's which resulted in burning a lot more calories without trying.
As we get older assuming your hormone profile has remained similar, I don't think your BMR really changes much if at all, but we do spend a lot more time resting/sitting/whatever just because we're old fucks.
Studies bear this out also.
Total daily energy expenditure ("total expenditure") reflects daily energy needs and is a critical variable in human health and physiology, but its trajectory over the life course is poorly studied. We analyzed a large, diverse database of total expenditure measured by the doubly labeled water...
pubmed.ncbi.nlm.nih.gov
Also, I don't see how people don't adjust what they're doing/eating before they gain 100 pounds over a decade. It's not like that just sneaks up on you. If you gain 10-15 pounds do you not go "I need to change my habits/do something" ??
