What happens to the body when one "fills out"?

What exactly happens to the body when one “fills out”? I’m talking about how one’s body supposedly changes from when one is 20 something to when one is 30 something – thicker chest, etc. Or are the changes simply a result of gaining weight from fat?

Do skeletal structures change? How could they at that point?

I am curious because I am in my mid 30’s yet sometimes still mistaken for being in college. I am in pretty decent shape, with low body fat, and still weight what I did in college, but I wonder if it goes beyond this. My friend recently observed that I don’t seem to have “filled out”. So is that a good thing (no gain of fat) or a bad thing (underdevelopment of the frame?)

Most people mean the addition of muscle when they say “filled out”. That happens to a few guys in high school (who are probably on the football and wrestling teams), but to more of them a little after. It seems to me that most boys “fill out” by age 26 or so with all the muscle they’re going to get without working hard at it, and after that they start to soften up with added fat.

Of course, there are always very slender guys who don’t ever “fill out” by adding much in the way of muscle, but they can still get soft with the added fat over the belly in their thirties!

Your skeleton doesn’t grow (at least, not significantly) once you are mature. However, if one exercises, especially the chest, shoulders, and back, the upper torso can become very noticeably thicker and wider due to the addition of muscle.

So then, once you are past the lanky and awkward adolescent years, there is no real filling out other than weight gain from fat? There is no difference beween the frame of a 25 and 35 year old?

I don’t think your bones are going to get any longer, and while they might get marginally thicker, that would be due to stressing them with exercise, which is going to add muscle- that will make you “fill out.” I’m 50, and I’m pretty small-framed, but I work out, and that has significantly broadened my shoulders and deepened my chest.

You mean after the muscle? You keep going back to fat, but fat isn’t what “fills out” a young man, muscle is.

But in a healthy person, no, there are no major skeletal (“frame”) changes after adolescence. There are some medical conditions, like COPD, which can cause what we call a “barrel chest”, which means the anterior to posterior, or front to back, diameter of your ribcage increases with age and the progression of the disease. Even that’s not really a direct result of bone changes, but rather the lungs never fully deflate, and the ribs sort of get “stuck” in the expanded position. Osteoarthritis can also cause the ribs to become fixed in the expanded position, creating a barrel chest.

Bones stop growing in your early 20’s.

Yes, but aren’t you pretty much as muscular as you are going to get in your early to mid 20s? I wrestled in college; most of the wrestlers on the team were very muscular and they were 22 at the oldest. I doubt they were gaining much muscle after college. I would say I physically peaked a year or two after that as far as strength was concerned. So now we’re talking 24, 25. Any “filling out” after that wouldn’t be muscle…we’ve ruled out skeletal changes…what is left but fat?

You can easily put on muscle well past 20. I am much more muscular at 50 than I was at 20. All it takes is hard work, and good nutrition.

You misunderstand me. Of course you can put on muscle after 25 by working out. But 25 is a good age for physically peaking in many ways. That is, you are not going to naturally “fill out” with muscle after that age. If you have already been training hard, you might hit your best numbers at that age in many sports.

I doubt you were working out as sensibly when you were in your 20s as you are now.

A smaller framed boxer is a good example. IIRC Sugar Ray Leonard was a light welterweight in the Olympics and early in his pro career. He was just a kid, barely 20 years old. He moved up to welterweight and held that title for several years. Then towards the end he was a junior middleweight. IIRC he even briefly fought as a middleweight.

He never got “fat”. Sugar Ray filled out as he matured into a man in his thirties. He always came into the ring in shape, but it’s just natural for a man to get more stocky as he approaches middle age.

Looking at Sugar Rays record…
he even fought as a Light Heavyweight and Super Middleweight.

His weight changed a lot as he aged. He could starve & train himself back down to welterweight, but it cost him a lot of strength. His last fight was against a middleweight champion, Héctor Camacho.

Yes, exactly. That’s the stage of “filling out” and it’s pretty much done by 25-26. The body changes in your thirties aren’t “filling out”, they’re “getting fat” or they’re “bodybuilding”, depending on what exactly you’re talking about and how much effort you’re putting in.

Just my observations from years as a high school teacher, and forgive the elementary, I-took-health-class-too information: during puberty, boys typically develop broader shoulders and somewhat narrow hips. At approximately the same time, they also experience a growth spurt; so while they’re shoulders and chests are bigger, they often look kind of scrawny. Once the growth spurt stops or slows down, they tend to gain weight, in both muscle and fat (not the unsightly fat, necessarily, just normal flesh). Even slower developers are almost always through this phase by age 20 or so. That’s what I’ve always heard referred to as “filling out.”

My guess is that you have a more slender skeleton and, with your low percentage of body fat, appear to have the frame of an adolescent male. (Think Topher Grace, who played Eric Foreman on That Seventies Show.) Your face may be more mature, though, so people tend to think you’re in your twenties. Just a guess.