It’s hard to say for sure because a secretarial or PA job is primarily a relationship management job with boss, other secretaries, boss’s stakeholders and external vendors. The basic admin stuff is pretty standard anyway (how old bird can you get from booking a hotel room or calendar?), so in theory anyone can do because it requires no specific technical qualification or experience.

From experience, I would say roughly 2/3 work in some kind of admin job for many years before while the rest have HR, Corporate Affairs or Sales background. It isn’t a place for high fliers, so most of them are at least mid or late 30s by the time they reach that level.