i feel for you, OP, I am 52 and am told that i look way younger, which I don't care about but which i know makes people take me less seriously.
I think you got some great advice here on little tweaks you can do to your look. I wanted to add - i was just at a work meeting, we were about 30 people, probably 2/3 women, different ages. (I live in Israel which is probably less "formal" than the US and I also work in a non-profit which is anyway less formal. Just for example, my boss was wearing jeans and a sweater). with that, i noticed that some women looked like college students even though they are in their late 30s-early 40s - it's not *just* that they have baby faces, but that they are wearing sneakers, t shirts, not very polished hair, etc. OR very focused on the "trendy" outfits which can make you seem younger. and others (same ages) looked way more professional - they might be wearing the same jeans and tops but they wore the jeans with boots and added a blazer, had their hair pulled back to a polished pony tail, had their nails done, etc. so even if you are in a field where you wear jeans, you could still make yourself look more polished - i notice that black jeans look "better" than blue or faded, boots with a mediium heel look nice, a blazer, nails (even with a neutral color or light french manicure), light makeup, etc.
as for what to reply to that woman - I think you are being too apologetic. she was extremely rude to you. For someone to say "it was so adorable how hard I worked, that I almost came across like a "real grown-up" and I was just cute as a button with my pigtail (note, I have long hair which I wore in a braid, not pigtails!) working the grill at the fundraiser." to anyone but a 10 YO girl is beyond rude.
For your reply, if you haven't send it yet I would go with something like this:
I appreciate the thought behind Thank you for your email, however it came across as being patronizing. I know it may be hard to see me as such, but I am an adult, not "almost a real grown-up." It can be very discouraging when at every turn my youthful appearance is remarked on ahead of anything else about me. It makes me feel as though I am viewed as a child unlike other 30+ year olds that find themselves in a different body. And, even if I wasn't an adult, my contributions to the barbecue stand on their own. I don't want this to turn into drama, but I need to share with you how I feel. I'd like to just move on from here and continue on as colleagues, with the mutual goal of benefiting our community."