The engineering community has a lot of trouble talking about seniority and what we consider the qualifications for becoming a Senior Engineer.
It's not uncommon to see folks scoff at the idea that people with under a decade of experience (or some other arbitrary threshold) can really be Senior Engineers™️; after all, how is it logical that two people with a potential 20+ year experience gap have the same title? Lets talk about it.
There's No Standard for Seniority
The most obvious thing to address first is that seniority is an ambiguous concept. Engineering culture, and the needs of the company, are what define seniority and that varies heavily across the industry. At a small startup you might have a Senior Engineer doing work that a Staff or Principal Engineer at a larger company would be responsible for.
Senior isn't the Most Senior
The title Senior probably carries more gravitas than the role typically deserves. While it's an ambiguous concept there's at least some agreement between the largest of engineering orgs about what senior means. You can look at levels.fyi to get a rough idea of how levels at the largest companies might compare. The interesting thing to note is that the levels considered Senior are typically in the middle of the leveling hierarchy. Google's L5 level comes with the Senior SWE title, but there are still 5 more engineering levels above that!
The idea that becoming a Senior Engineer early leaves you without mobility doesn't hold much weight. In a mature engineering org there is often plenty of room to move up from a Senior Engineer. Becoming a Senior Engineer is not the final act of an engineering career, though it often can be if that's what you want.
Many large engineering organizations have what is called the career level or terminal level: the engineering level where you are no longer required to progress to the next level. This is almost always one of the first Senior levels. When I was at Facebook this was E5. At levels below that there was an expectation that you would mature and level up over some standardized timeframe, but once you hit that career level you can stay there your entire career if you'd like.
This is why it's expected behavior to see folks at this level with varying years of experience; someone might become a Senior Engineer early on and decide to stay there for a giant chunk of their career.
Growth Rates Differ
Lastly, we should just recognize that some folks develop the skills required of Senior Engineers faster than others. This might be due to an inherit disposition for the work, past experience from another career, or any number of factors. At best years of experience is a proxy metric for figuring out if someone might have had time to develop those skills, but like most metrics we can't depend on it giving us the whole picture.