Hello,
I wondered, to acquire that much skills the candidate must have min 15+ years experience in various areas
Yes, often senior sysprogs have much more than 15 years. Many have more than 25 years. . .
And as Mongan mentioned - some small organizations may have only 1 or a few sysprogs. There is also the other end of the rainbow. . .
One of my recent clients has a "Technical Support" staff of over 125 people (which includes Operating Systems, CICS, several Database Management systems, Storage Management, and Security Administration). This organization runs 12 z8/z10 boxes with 36 or so lpars and 350+ cics regions (the numbers go up every few months). There are more than 200 terabytes of data spinning on the floor and more than 1/2 is for databases. Before the migration/comversion to V-tape, the "tape" inventory was over 2 million volumes.
IIRC, no one in Tech had less than 10 years experience - most had more than 20. When they hired (or rented as in my case - there were several contractors as a requirement can usually be met quickly by bringing in a contractor and not going the entire interview/hiring process) they needed someone who would be productive as soon as they had id's/pwds