The problem isn't that developers are building units that appeal only to young people, the problem is when they're exclusively building those units.
We are building a smaller apartment building in Dallas and everything from unit numbers, to sizes and shape of building is driven by the zoning rules. And because the land is sold based on those rules you can’t really make it different otherwise the project isn’t viable anymore. Thats why everyone is currently building townhomes and not small apartments buildings in places like Old East Dallas or Bishop Arts District.
I lived in one of those at one point in the Seattle area and there were families (mostly with small children) in them.
We have same issue in my city. New apartments are 40-50sqm 2 rooms (1 bedroom + kitchen-living room) or 60-70sqm 3 rooms (2 bedrooms + kitchen-living room ) at best. If you want family-sized housing, house in suburbia is pretty much the only option.
Sadly many apartments are not built to withstand a screaming child in a neighboring unit.
When the children are older those things don’t matter quite as much, but by that time people often can afford to move to more space.
The bigger the landlord is, the harder it could be, but for single family homes, if you're renting and want to add a deck and are willing to pay, talk to your landlord. There are risks (landlord evicts you right after you build it, etc) but those can be mitigated.
And if there are no sound-dampening apartments available in a particular area and price range, then the apartments you're trying to rent out not having them isn't a competitive disadvantage, so why bother?
Now city government acts surprised why developers build so nice tiny apartments, yet suburbia is growing at fast pace. Who could think that if you give out permits for mostly tiny apartments in gigantic apartment blocks, people will find other ways and try to fix the problem in rather anarchistic ways.