As someone who heard from EA at age 40 and is now 50, this question comes up often in discussions with more experienced professionals. I wrote about my personal journey, which I couldn't have done without the luxury of much free time to learn and explore in this space. A recent post by Jim Chapman also describes the effort that can be needed to transition into the space.

In my local group, I'm mostly the oldest participant, as many other people my age don't feel very welcome in a group of younger people. They mostly also see donations as their pathway to contributing, which doesn't require the level of involvement in the community that people with career ambitions have. The group is simply less useful in an instrumental way; the same applies to EA conferences.

In organizations, the hiring process can sometimes be more focused on younger candidates. Some organizations prioritize a strong alignment of values, which involves cultivating hard-to-fake signals such as investing in networking, volunteering, attending conferences and retreats, and making a pledge that may present a significant challenge for individuals with family responsibilities. 

The first time I applied for jobs was in my late 40s in EA organizations, as I was previously accustomed to networking and receiving invitations for work opportunities. Completing work trials under time pressure often seemed tailored to those with more fluid intelligence, which is typically higher in younger individuals, as opposed to the crystallized intelligence that develops later in life. 

Sometimes, experienced professionals will vent how they were invited to a job and then had to start at the first stage, how they were treated unprofessionally in the hiring process, or even when they start how their expertise is not valued in an organization led by people with little prior leadership experience. This can lead to losing out on more experienced people. (At Successif, we help mid-career and more senior people navigate these challenges in the area of AI risk).

This leads me to the question if EA is the right place for more senior people. When I talk to people my age about impact, I'm more likely to recommend the donation opportunities at Effektiv Spenden, the 10% Pledge, or the book Moral Ambition for career inspiration than the global or national EA websites. While I often enjoy being the oldest person and spending much time in deep discussions with philosophically minded people 20 years my junior, I expect this to be the exception. People with families and busy jobs are probably looking for a quick way to shift their focus and connect to people. at a similar point in their life Other services and brands are probably now better suited to cater to this need than EA.

One project aimed at mid-career people: https://www.successif.org/our-work

I expect there are some cohort effects (people in more recent generations have a higher probability of being involved).  In particular, many people get into EA via university groups (although it may not be the place they 'first heard' about it; see David Moss' reply), and these groups have only been around a decade or so. 

But I also imagine some pure age effects (as people age they leave/are less likely to enter), perhaps driven by things like 

1. Homophily/identity/herding: If you only see people unlike you (agewise) you're less likely to think you belong. This leads to inertia. 

2. Cost and family priorities: EA ~expects/encourages people to donate a substantial share of their income, or do directly impactful work (which may be less remunerative or secure). For older people the donation share/lost income could seem more substantial, esp. if they are used to their lifecycle. Or probably more significantly, for parents it may be harder to do what seems like 'taking money away from their children. 

3. Status and prestige issues: EA leaders tend to be young, EA doesn't value seniority or credentials as much (which is probably a good thing). But older people might feel ~disrespected by this. Or second order: they might think that their age-peers and colleagues will think less of them if they are following or 'taking direction' from ~'a bunch of kids'. E.g., as a jr. professor at an academic conference if you are seated at the grad students' table you might feel insecure.

4. Issues and expertise that are relevant tends to be 'new stuff' that older people won't have learned or won't be familiar with. AI Safety is the biggest one, but there are other examples like Bayesian approaches.

(Identity politics bit: I'm 48 years old, and some of this is based on my own impressions, but not all of it.)
 

I expect that some of the older EA's are more senior and therefore have more responsibilities competing against attending EA Global.

Looking at EA by movement evolution, it appears to me that EA remains quite conceptual exclusive. EA currently operates with relatively strict ideological boundaries:

• Intellectual requirements: EA heavily emphasizes rationality, quantitative reasoning, and philosophical frameworks that can create high barriers to entry.
• Conceptual vocabulary form an insider language that can be exclusionary.
• EA's roots in universities and philosophical traditions naturally filter for younger, academically-inclined participants.
• The dominance of Western, particularly Anglo-American perspectives creates barriers.

I think this exclusivity — as you can observe in EA post like this one or in the application for EAG —  could explain the age demographics you've observed. Older people with more established worldviews may find EA's strict frameworks less appealing than younger people still forming their philosophical thoughts.

For EA to mature as a movement and achieve broader impact, I think it could transition to a more inclusive approach (creating space for those who share EA's core values but approach them through different frameworks, traditions or cultural contexts). I do agree that it is difficult to maintain intellectual coherence while broadening some thoughts. But it might be an explanation.