I’m just asking around people in the voltron fandom bc I’m not sure of this but in an AU where she’s a human how old do you think that Haggar is?
I’d say... take whatever age you gave the middle paladins (Lance, Hunk, Keith), and add about thirty-five years. If they’re elementary schoolers, she’s could be in her early forties. If they’re college students, she’s maybe somewhere around fifty.
Granted, this goes off the assumption that you’re making Lotor about five years older than them. If you’ve already picked an age for Lotor, then I’d say add about thirty years. As Honerva, Haggar never looked much older than her mid-to-late twenties until just before she died, and Alfor mentions that it took decaphoebs for her to go from the “sensible but cold” of Honerva-in-Empress-clothes to “quintessence-mad and wearing robes,” Alteans live a long time. Granted, Alfor was implied to be hitting his stride as... what, about forty? Since Allura was already a teenager? But Honerva was also, I think, framed as being a little younger than him.
So I’d say Honerva was probably around the Altean equivalent of thirty or thirty-five when she died (you could probably push it up to about forty if you felt like it), and we’ve had it confirmed that she was pregnant at that time and that Lotor got some rift exposure that way, so Haggar is the in-universe equivalent of thirty-five-ish years older than Lotor, who is... however much older than the paladins you want him to be, I guess? Depends on the AU; if it’s a high-school AU, he’s probably not more than three years older than them; if they’re already in college or adults, you have more wiggle room to include him with a larger age gap without making it weird for him to be involved in the plot.
So yeah. Take Lotor’s in-universe age and add about thirty or thirty-five years.