I know the question is about ageing in 30s / 40s... but just fast forward a few decades...
I work with older people. A good proportion of the people I visit tell me that they don't look their age. They almost always do look their age (which is a normal, positive, healthy thing). But they tell me, proudly, that they don't. I don't initiate the discussion, they do. The conversation usually goes something like this: "The nice young lad from John Lewis said he couldn't believe I'm 78". Or some variation of that. A lot of the ladies actively seek out the affirmation. So, a workman will call at the house, or they'll be in a taxi, or other such situation, and they'll say pointedly, "I'm 78 you know", and it hangs in the air awkwardly, until the workman or taxi driver or whomever responds with the required "eeee you don't look it". By no means all of the people I visit do this, a lot of the time it doesn't come up at all! But it's surprisingly common. And where it does happen, almost all of the ladies definitely do look their age, they just can't (or won't) see it in themselves, and want affirmation that they don't. And will immediately warm to the lovely salesman who flatters them. It makes people incredibly vulnerable, and it's really troubling to see.
I'm convinced that a similar pattern exists among younger folk too.
Being asked for ID in a shop, or having someone say "eee you don't look 40 " (or whatever) doesn't necessarily mean that's true. Some staff are jobsworths. Some people are "buttering you up". Sometimes it's 100% genuine, and we all know people who who look younger than their age. But most people don't.
Fair enough, some people age faster than others. Lifestyle, genetics, etc etc. But even taking those things into consideration, we're probably talking about a few years either side of actual biological age. Most people look in the rough vicinity of their age. And I really don't understand why that's a bad thing.
Why is the OP confused and sad that a 30 year old doesn't look like a teenager? Why do they think it's acceptable to judge a friend on the basis of how old they look? And why is it a negative thing to look older than you did ten years ago? Bananas!