But being older doesn't mean you stop learning, adapting to change, or lose the ability to deal with moving targets. I'm hopping rapidly between roles and widely varied tasks at my current gig better than I would have in my 20s or 30s since I've got more perspective and experience.
A more mature person might point out that if your plans change weekly you likely have a systemic problem, though, and would be more likely to point out the breakage.
This stuff is a false economy anyhow since it inevitably leads to a massive ramp up in technical debt even with the very best programmers.
I am not suggesting that you don't have a problem if your plans change weekly, but unfortunately this is the way many businesses are run. When you have a very fluid workflow youth is very attractive.
i.e. you don't bother to spend an hour to plan a project. I don't think anyone wants to work at those kinds of places, young or old.
But flexibility is a hard-to-measure parameter to begin with and you need to know the people and their reasons before you can make call about it rather than simply their age.
In general, if the plan is changing every week, that probably means that the work I did last week was mostly pointless. That's a great way to burn someone out.
Good news though! I do contract work, and if you insist on changing the plan every week, you won't need to worry about whether I'm flexible or inflexible. I'll happily fire a client who can't make up their mind and thinks that the best path is to change direction every week.
But I also know with experience comes a skeptical eye for being asked to carry extra load for less upside (yet same or more risk).
A story from some time back told to me. Founders had a UI, an algorithm, and an idea. It had merit. They sought someone out to build and run the actual service. The funding was angel, the role was senior to C-level, but the salary offered was 40-60k cut for a senior engineer at the usual suspects, equity capped at 4% assuming a trade off in salary.
Risk and reward to build out infrastructure and productize someone's idea? I could certainly get a couple of people to build it out in a "get us to a-round funding" in those guidelines, maybe even beyond. But as expectations on a non-founding senior/early member, one would assume way too much risk for the potential upside.
It isn't non-technical founder seeking technical co-founder waters, but damn close.
For those who use their minds, which should be common in this field, those effects become noticeable much later in life. For most of the working lifespan the dominant effect will be an increase in experience.