The reasoning is very simple Stevie - the wealth and the productivity of a nation essentially is dependent on the proportion of its working population, compared to its non-working population.

The largest sections of the non-working population are the young and the old - the former having the potential to become producers whilst the latter will remain as consumers.  The largest number of producers come from the 25 to 65 age group.

The ratio of immigrants in the 25 to 65 age group is far greater than that in the overall population.