Cani shoot steel casing with brass heads of 9mm at shoot indoors?

I think i recall buckshot being the only restriction there

They don't care about the casing, just the slug. They will test for steel cores by touching a magnet to the tip for you at the range or you can test yourself using the same method.

Hold a magnet to the bullet and if it sticks then there's steel in it and you can't use it there

I would say no. Anything that is picked up by a magnet is not acceptable for the range..

Any age limit

Steel case and brass case is ok but no steel bullets or steel penetrators

Yes, they are fine. I shot Winchester steel case with lead bullets without issue.

I think steel casings with brass going down range will probably be ok