I expect to get my PPL soon and serioulsy consider to build an
airplane simply because it's cheaper.

If you are considering just the cost of the plane it might be cheaper. But if
you factor in all of the tools and heated/airconditioned shop needed to build a
plane the cost rises quickely.

I am in
20s and I read the average age of kit plane builders is over 50--this
scares me.)

Earning the money for, and acquiring the above mentioned tools takes time for
most of us. And if you decide to have a family along the way life kind of has a
way of taking your time...........

I like to build things and would have bought the tools and shop anyway. But if
you want to fly, buy. I could have bought several nice planes for what I have
invested in my hobby.