We were advised to start bedtime routine at 6 weeks for our first baby.
The advice was to do a 7/7:30 bath, quiet, dimly lit, feed, and then settle into the cot (nowhere near a TV).
He was a particularly unsettled and a round-the-clock feeder, so there was no let up and we were desperate!
Initially we poo-pooed the idea (a routine? For a newborn?!!!!) but tried it anyway, and were amazed at the results.
He settled better and slept longer! We had been warned it wouldn't happen on day 1, but it actually did. So a huge success for us.
However, I am currently with a newborn, a prem baby who is 10 weeks old (adjusted age of 5 weeks), and who is nowhere near a routine at all. This one is impossible to even take upstairs to settle in the cot, and we take it in turns to do a shift downstairs, so the other person can sleep through the disturbed night.
So my point is, every baby is different and a routine can be a God-send, but not for a tongue-tied, refluxy baby like the one I've got! Maybe it's a case of "suck it and see", be happy if it helps but hold it lightly in case it doesn't.
As for babies sleeping through at 12 weeks, mine didn't, and I can safely say the second won't either.