As someone who has lost 10st, this is my take on it:
I believe that people categorise you in the 'fat' category.Then once you lose weight, there's a moment where you don't fit into that category. So when you "don't look fat" out come the comments. I suspect that happened to OP. I can imagine that being a size 16, I probably do look gaunt when the last time you saw me I was a size 28. But you get compared to the old you, not what being healthy is.
It's similar to people whom you suddenly see without their glasses. Their face looks odd. But ...that's their face! That's what they look like. But you're not used to it.
I feel sad that there are so many people here having to justify their weightloss and justify that they still need to lose weight etc. Especially when it's no ones business. That can feel quite demeaning, to express how fat you still are, so that someone can lay off the concerned comments. 🙄
RE the ageing - I'd love to advise weight gain as an anti-aging tip, if that's truly what happens.😅
I can name several thin people who look aged, gaunt, no curves, no bum, etc but we aren't 'concerned' because we have never seen them overweight.