This is a how long is a piece of string question.
It's all about build quality, age is only semi important.
Old buildings, and by old I mean <1930s tend to use overly thick solid walls, now this can be good for sound proofing, but also bad.
Modern building methods require a separate party wall (an actual air space) between the two adjoining properties, as well as mass to block sound. The mass blocks/subdues it, the air space makes it more difficult to bridge and spreads the sound.
Old houses don't have party walls with air spaces, but do just have a heck of a lot more mass, this does mean however any "break" in the wall makes them very prone to transmit sound
For instance a poorly fitted joist spanning through the wall, I had this issue with an old property with solid walls, at the part of the wall it spanned through, you could hear sound bleed, footsteps or voices etc, luckily the part of the wall in question was opposite corridors on both sides and not living spaces, so there wasn't much sound to pass through.
Modern building regs do have acoustic requirements, and there are design principles to easily meet them, very basically you should not be able to hear people speaking VERY loudly (just under a shout).
The problem is that it's easy to cut corners and b*****r up the acoustic details, and it's something to easy to get away with.
If you are really worried about it, go along with a friend and have them go into the adjoining property (I assume it's a new build estate, so there are plenty of empty houses?), get them to shout or bring along a getto blaster
.
You can also look up "robust details" on google, that has some handy acoustic details.