I think there’s a lot of evidence that for most things (like “new tricks”) there isn’t any learning disadvantage for older dogs.

However, leash reactivity/aggression is not about teaching a dog a new trick (or even a new behavior) as much as it is trying to change an underlying emotional response to stimuli. I don’t know anything about your dog, and there are lots of reasons why dogs can react like that on leash (fear, intense desire to play and socialize, actual aggression), but usually if the dog is freaking out they are past the point where the “logical” part of their brain can make decisions for them.

The human analogy is that people of all ages can learn new skills, like how to cook a new recipe or build a cabinet or something. But if a person has a fear of heights, they can’t just learn the skill of not being scared of heights. That requires rewiring the base emotional response, which takes time and has a high rate of failure.

For dogs I worked with, we usually asked ourselves if we thought the dogs reactivity was lowering their quality of life. That is to say, does your dog need to be calm on leash to live a happy life? If you live in a city and she needs to navigate past dogs every time she goes to the bathroom, I’d say it’s worth trying to make that less stressful for her. If she lives in the suburbs or in a rural area, and she barely ever encounters other dogs anyway, why bother? The human analogy is: if a person works on skyscrapers for a living, they probably need to not be scared of heights, but many people are scared of heights and live totally fine and happy lives from the ground. Of course, a human can have some control over their environment and career, but dogs don’t have that luxury. We work with them where they are.

It’s also worth noting that even the best “rewiring methods” take a long time and usually only work to reduce the fear, not eliminate it. We do it because it will really improve the dog’s quality of life, not because we (selfishly) want a perfectly behaved dog to bring everywhere and do everything with.

A side note is that there are critical periods for dog socialization and development just like in humans. I haven’t looked into this in a while but me memory is that it occurs at like 8-12 weeks of age. Puppies in this period learn about how to interact and communicate with other other dogs in the same way we learn language early in life. If they aren’t socialized in this period, they usually struggle to effectively socialize for the rest of their lives. Still, this isn’t necessarily the cause of leash reactivity, so I’m just throwing it in as a side comment if you want to learn more.

AND lastly, if you do decide you need or want to work on your dog’s reactivity, I strongly recommend Grisha Stewart’s BAT 2.0. I am not affiliated with her in any way, but this is the technique recommended by reinforcement-based training organizations and has the most likelihood of reducing reactivity in my personal experience. The textbook is like literally hundreds of pages long and covers a ton of case-scenarios. It would take time to read and learn to do the protocol, and you need a BAT leash (a 15 ft leash) and lots of practice managing it, but I have seen dogs go from freaking out and screaming from seeing another dog >100 yrds away to being able to (tensely) stand to the side of a sidewalk and contain their panic as a dog passes. It won’t make a reactive dog confident and bomb proof, but it can make a huge difference when applied carefully and consistently, especially with other methods like look-at-that/counterconditioning, Karen Overall’s calmness protocol, and engage-disengage games.