The belief that economics is a zero-sum game is sadly quite prevalent.

It's not about zero sum.

It's about supply and demand.

If you supply labor at X skill level, an influx of immigrants at X skill level (but not of Y or Z skill level) will reduce the buying power of your wages.

Taking it to real life - people tend to oppose immigration when they are at X skill level, and support it when they are at Y or Z skill level. It's raw self-interest often dressed up as altruism or principle.

You can see this understanding here whenever H1B is discussed - suddenly supply and demand becomes an obvious economic concept.

> so you have merely increased the population.

And often they're in they're twenties, meaning society didn't have to pay for the expensive first two decades of growing up. Basically a live is 8 decades, 4 decades of which are unproductive, and 4 decades are productive. If you get an immigrant, you get a 6 decades person that's unproductive for only 2 decades -- and some other society paid for those 2 other unproductive decades.

Median age for immigrants to America is 44.

Median age for all Americans is 36.

Sources:

http://www.migrationpolicy.org/article/frequently-requested-...

http://www.pewhispanic.org/2015/09/28/chapter-5-u-s-foreign-...

Thank you for backing up my point with some numbers. In particular see the second link, the age pyramid, showing how there are very few people in the foreign-born population who are younger than 20.

And indeed, when you remove most of the population who are younger than twenty, the medium age of the group overall will increase.