This is a very difficult topic. Since discriminating on age is illegal in most countries, there aren't (much) sites/platforms where age is an actual explicit factor. Generally speaking, startups/scaleups are more inclined to hire younger folks. Older companies are more open to hire older engineers.
The bypass might be, looking for companies that have a lot of engineers with 15+ years of experience. You can do so with paid LinkedIn accounts. I'm happy to help with some specific searches if you don't have a paid LinkedIn account yourself.
You might be right that's why they don't exist, but if so I'm not sure it's well-founded. The same legislation prevents discriminating on gender, but there's no problem (that I'm aware of, and there seem to be plenty of them) with women-specific recruitment/careers things.
You can't use age (resp. gender) as a factor in your decision making (bloody difficult to prove one way or the other most of the time I imagine) but that doesn't mean you can't run jobs4oldies.com or Women's Careers Day with Special Guest Sheryl McHighflyer.
That doesn't mean you should, but I'm pretty sure it's not illegal (in the UK, or generally against the spirit of similar legislation). IANAL.
It's indeed not illegal to do so. I'm just trying to explain why most job boards/ platforms don't register/work with age. Why there are a lot of initiatives around woman in tech and not oldies in tech? I'm afraid the answer is; there's a lot of karmapoints to be won first the former, while the latter will hurt your image/brand as the next best thing where magic happens.
(Not saying I agree with this or I like it, it's just what I see).
In the US, it's only illegal to discriminate based on age of 40 or older, so it would be perfectly legal to run an ad that says "under 40 need not apply".
The bypass might be, looking for companies that have a lot of engineers with 15+ years of experience. You can do so with paid LinkedIn accounts. I'm happy to help with some specific searches if you don't have a paid LinkedIn account yourself.