cities-skylines
I noticed that after a certain size, around 30k population, my city's population growth rate declines a little without expanding residential zones. I then briefly looked around the houses in my residential districts and saw that a good number of homes had seniors as residents. This was around 20 years into the game from the start.
How long does it take for a person to grow up?
There are two factors to educational facilities. First their is coverage, also called efficiency. This is shown on the map by the colour of the streets of your city. Secondly, there is capacity. There are three different educational infrastructures with capacities of 300, 1200, and 4500 respectively. The capacity correlates with their coverage (the more capacity, the larger their coverage area).
When you place a new establishment, you want to look for the street's colour. The greener it is, the more efficient the school will be for that area, that is, the green area is covered by the school. When you investigate your city using the Info Views (top left-hand corner, second element on the right-hand column), there are three tabs, one for each educational building. You can then also examine the placement of schools by looking at the street colours, and see the availability, which basically just tells you how many of the available capacity is taken by your current citizens.
Finally, you may also want to have a look at a video by SergiuHellDragoonHQ on Youtube which will teach you about Education as well.
I don't think its possible to see how much income a specific building generates. The highest level of detail you can see is the weekly income for zones and levels of a specific building type. You can view this information in the Economy menu specifically by hovering over the type of interest.
For example here is the weekly income breakdown for industrial levels and specializations.
Best Answer
It takes about 2 years of game time for your first senior to appear in game. My first senior appeared after about 2 years of the first citizen moving in. I do not know if its is a variable amount but that's what I experienced while testing.