Is there a way to view a map the density of persons under or over a certain age by census block?

Factual Questions
1

I’ve seen such maps before but I don’t know if this is publicly available.

2

This might help; it requires Flash so I wasn’t able to dig very deep.

https://www.census.gov/geography/interactive-maps.html

3

Alternatively you can download the free open-source GIS mapping software QGIS, and do it yourself with publicly available census / ACS data.

There’s a bit of a learning curve regarding projection systems and how to get the software to do what you want to do, but it’s very capable and it may be worth going this route if you’re going to want to dig into such data more than once.