How do people practise in a flat?
A digital instrument with headphones for volume, and a rug plus felt pads for the structure-borne noise that headphones do not solve. The complaints are usually about thumping through the floor rather than the sound in the air.
Is it realistic to start piano at 35?
Yes, with the caveat that adults quit for reasons children do not: impatience with the first plateau, and practice sessions scheduled at the end of the day when willpower is gone. Twenty minutes in the morning beats an hour promised for the evening.
Anything that changes this at ten times the scale?
At ten times the size the bottleneck moves from the operation itself to what it competes with: memory, locks and the people who have to run it. The approach stays, the batching gets smaller and the schedule matters more.