Use it or lose it.
I'm 53.
I can't lift as heavy as I used to, but I make it a priority to at least maintain the ability to maintain core movement patterns (lunges, ATG squats, side lunges, cossack squats, 1 leg RDLs, overhead) etc.
Even some light plyo.
I agree and don't agree, I guess. There are some physical skills that are probably going to, ultimately, end up in the trash heap as we age. I loved basketball and riding a bike when I was younger - I haven't done either in years and they aren't priorities anymore. More germane to the topic - I've hurt myself many times now w. (Bulgarian) Split SQs over the years that I'm starting to actually side with Pavel when he was really anti-lunge and Bulgarian Split SQ in a conversation I had with him like 15 years ago (I don't claim to know how he feels about them now).
Anywho, I don't see a burning NEED to do lunges/Bulgarian split squats specifically, unless, of course, it's what the OP really wants to do. I feel like loaded carries/sled, hill work, etc probably fill the gaps left with only bilateral squats and squat variants.
I'm not a huge TGU fan but if someone wants to include lunges (without really including lunges) then the TGU does the job and certainly slows things down a lot.
edit: I'm sorry - after re-reading the original post, it sounds like lunges are exactly what the OP wants to do. IN THAT CASE, ignore most of what I said above. If it were me, I'd be doing some kneeling and tall kneeling work (presses, stretches, glute/ab work) to feel comfortable in those positions, mobility and strength work to make sure the legs, hips, etc are up to the task, then work on some progressions to get to full lunges again (front foot elevated, shorter rom, TRX assisted, etc). And, losing some weight is probably a good idea for the joints overall and will make relative strength/bodyweight work easier.
