IQ doesn't decline with age. Pretty simply because IQ is deliberately determined in an age agnostic way: your IQ score tells how well your problem solving skills are compared to your age group.
Fluid intelligence does decline with age, but most people are said to be able to keep up fairly decently by utilizing (and building) crystalized intelligence. Now, of course, your usefulness as an engineer may very well decline with age. Though there are different roles that you can take on in most organizations. It's anecdotal, but my mother used to be a software engineer until her retirement in her sixties, as well as her colleagues she used to work with when I was a child. They started their careers in the 70s and 80s and most of them never left the field.
Up until around age 35 or so, I was generally very lean and did not get noticably fat regardless of my lifestyle.
I know I haven't gotten any stupider. Hormonal changes are the most likely difference for me.
> "Up until around age 35 or so, I was generally very lean and did not get noticably fat regardless of my lifestyle."
It's tempting to read this as saying "I didn't choose it to happen, so I'm excused from any negative effects", although that would be taking a negative view on it. But if not that, what else is this sentence here for? Maybe you're saying that there's more to fat than lifestyle - then skim read "Ten Putative Contributors to the Obesity Epidemic"[1] (PDF or online PDF) where they discuss things like viral infections, environmental triggers, socieo-economic contributers, and other things which aren't diet or exercise related. Does any of that change whether obesity does or doesn't correlate with IQ? Or maybe you're saying you got fat despite exercising - which only says exercise didn't save you, it doesn't need to change whether IQ and obesity are correlated or not. Plus exercise was suggested as a counter to age related decline, not as a counter to fat, only as a thing people might think when seeing fat.
> "I know I haven't gotten any stupider."
Who ever thinks they have? Would you honestly know? You ought to have lost some abilities just going from age 35 to 55 alone. See[2]: "Age and individual productivity: a literature survey" - "The ability levels of employed white men and women up to the age of 65, using data from the General Aptitude Test Battery collected in the U.S. from 1970-1984, is shown in Figure 2. These findings suggest a relatively sharp decline in most abilities, after maximum values are reached in the 20s and early 30s (Avolio and Waldman 1994). The decline of mental abilities from early adulthood is a universal phenomenon. The age-induced changes in cognitive abilities are similar across countries and within population subgroups, such as between men and women (Park et al. 1999, Maitland et al. 2000). Further individuals with high and low ability levels are subject to the same age-induced changes in cognitive functioning (Deary et al. 2000). Even among non-human species, ranging from fruit flies to primates, age-reductions in memory and learning capabilities have been observed (Minois and Bourg 1997, Bunk 2000)."
[1] https://www.researchgate.net/publication/40447388_Ten_Putati...
[2] https://www.demogr.mpg.de/papers/working/wp-2003-028.pdf
If lower IQ correlates with obesity ("a 10‐point decrease in IQ was associated with a 1.10‐fold increase in the odds for obesity."[2]) either way - a lower IQ person is more likely to get fat, or the metabolic changes of getting fat negatively affect cognition as well - over a large group of skilled people would you expect thinner people to earn more?
If height and IQ are correlated ("Taller people tend to be smarter. Although the relationship is modest, height and IQ are consistently correlated at ∼.10–.20"[3]), would, etc.
[1] https://www.mdedge.com/neurology/article/193699/alzheimers-c...
[2] https://onlinelibrary.wiley.com/doi/10.1002/lim2.11
[3] https://journals.plos.org/plosgenetics/article?id=10.1371/jo...