There's no reason to be rude, you said you work in a place where you get to closely observe counselling patients, how was I supposed to psychically know that you actually work elsewhere in the building and just spend a lot of time spying on some unrelated business that happens to be in the same building?
Impressive that you manage to get any of your own work done, considering you're somehow able to keep such close track on other tenants' clients that you're able to monitor and gauge the age of every single person who comes into the building.
I actually have worked in mental health (my PhD was in the neuroscience of mental illness), and it's a very even split age-wise. That's based on having actual professional experience and access to national stats, not "well there's a counsellor in my building and I mainly see young people, so obviously this case study of one means only young people seek counselling." Perhaps the person in your building specialises in young people?
And mental illness has nothing to do with "not coping with stuff" or not having resilience. That's ignorant and stigmatising.
Do you have any idea just how many people were institutionalised for mental illness in the 1960s, 1950s, and before? How many people were on valium? What huge number of men were treated for PTSD (then named "shellshock") after WWI and WWII? My grandfather who served in WWI was treated for PTSD. I have immediate family members who spent time in psychiatric hospitals in the 1950s and 1960s.
Many, many people keep their mental health concerns private, or are in denial or simply don't recognise things as signs of a mental health issue. There's absolutely zero way you could say for certain that the person who lives next door to you has never experienced anything other than perfect mental health their entire life.