I mean, depends on what one means by 'understands'. Here is my definition of having understanding of a full stack:

I am nearly 50, started with computers when I was 7. I did sort of everything -from wiring individual transistors together into custom logic circuits, assembly on various platforms, drivers and other kernel level stuff on dos and then windows, low level networking, higher level monolithic systems design, infrastructure management, various storage systems, various compute implementations, obnoxious types of software defined networks, security stacks - from hardware based attestation through identity/role management in multi-cloud environments, higher level distributed systems and finally i am a cto for a large-ish company and do a lot of enterprise architecture.

I also dabble in hobby programming on various things, building custom firmwares for chinese electronics, building frontends and backends for side projects, I have built a js framework or two just to understand state and rendering stuff, and of course now i am dabbling with local LLMs, because thats the new thing that can be learned.

I am sure there are lots of people with similar experiences in my age bracket (I am 48) - who had a lot of exposure to all levels in the stack. Sure, I was never particularly good with anything I mentioned, but thanks to a mix of diverse work experiences and absolute, uncompromising love of learning new computer things, I think I do have a fairly good understanding of what happens in computer systems, big and small. Now, could I design, say, a modern ELB properly, no, but I do understand the networking stuff enough to at least outline the design and key elements of the system without asking a friendly LLM. No such luck for kids these days, I am afraid - they will be actively shielded from having to learn stuff the hard way.