I've gotten so much better over the years, as I myself get older. When I was in my 20s, I was at least a decade off when it came to guessing someone's age. I would think, "This lady must be in her mid-40s" only to find out later that she was well into her 50s. Now I'm actually pretty right on within a certain age range. There will always be exceptions, of course, based on a person's ethnicity, health, lifestyle, and aesthetic factors. I have a somewhat accurate technique that I always use: When in doubt, I look at the person's elbows (that's right) and see how wrinkled they are. As a guy, it's harder for me to guess a woman's weight than a man's weight because of differences in body and muscle composition. I would probably be able to guess someone's weight in terms of the range, as in 170s, 180s (lbs). As far as height, all I do is compare someone to my own height and figure out the difference.
I have trouble telling the height of tall people. Once they get over about 180 cm they are considerably taller than me (I am 159 cm tall, or 5ft 3in). I am very good at judging children's ages but less so with adults though, like you, I am much better than I was when I was younger. I think I can guess a woman's weight easier than a man's weight.
I compare people to me and usually mess things up. I'm 49 but I'm told I look 40. When I look in the mirror, knowing I'm 49, and then look at people who are my age, they look old to me; people who are younger look like kids to me. I'm only 5' 1" and am used to having to tilt my head up to look at most adults. I judge height only by quickly measuring myself against them. Visually, that is. I'm...let's say "chunky"...and my actual weight is more than it appears. By easily 20 or more pounds. So someone who is my weight is physically much bigger than me. Not sure how that happened. Even my doctor doesn't understand that one, but says I carry my weight well. But still need to lose some. Hubby, conversely, was always good at guessing age, weight, height, and even income where we used to work (in the 1980s). Amazingly/annoyingly observant.
It's probably safe to say that your own age plays a factor in how well you gauge someone's age. As a teen, I pretty much thought anyone in his/her 40s was "old." What I've observed is that as I got older, younger people seemed very young to me while older folks were not as old as I originally perceived. With due respect to everyone's background, it has been said that Asians (women in particular) look younger than their actual age . I've found this to be true among my mostly Chinese female coworkers. There's one in my department who could pass for a 25 year old woman. She's in her early 40s, married, and has two kids in high school. She's very slim, hasn't had any work done, and takes good care of her health.
I'm terrible at judging age. The few times I've made an assumption, I've been off by quite a bit. Luckily, it was usually judging someone to be younger than they were, which typically pleased them rather than causing discomfort.
In person I'm pretty good at all 3. Age is probably the hardest, lots of random factors involved. Height isn't hard if you compare yourself and surroundings. Weight is pretty easy based on body type, height, width, etc. RAMA
I'm terrible at guessing someone's age, because I've always looked younger than I am. My hair's starting to go a bit more silvery now, but I was carded as recently as two years ago (I'm 45). Height is not too difficult, because I can generally estimate the difference between my height and someone else's, with one caveat: if someone is outside the height range that's considered "normal", it gets harder to judge the difference. I'm acquainted with a midget, and if he hadn't once actually said that he was 3'4", I would probably guess anywhere between 3' and 4'. Similarly, once someone gets over 6'5", it's tougher to estimate. For weight, it's not too hard for me to estimate a man's weight, though if someone is more muscular, it tends to throw my estimate further off. I know that muscle is heavier than fat, but it throws me to learn that someone who's four inches shorter than me actually weighs more than I do (especially since I'm a bit heavier than I would like to be).
I'm fairly good with age. Honestly, I suck at estimating someone's weight by eye. It's a shame since it's part of my job. Then again, I don't think it particularly uncommon.