Learn French: Dès que cette génération aura vieilli, la démographie changera. - As soon as this generation has aged, the demography will change.

In French, when you use temporal conjunctions like dès que (as soon as) or quand (when) for events that haven't happened yet, you must use a future tense. Because the generation needs to finish aging before the demographics change, French uses the futur antérieur (future perfect) to show that this future action will be completed first. English typically just uses the present or present perfect here.

How exactly is the verb aura vieilli constructed?

It is in the futur antérieur. You form it by taking the futur simple of the helping verb (in this case, avoir, which becomes aura for cette génération) and adding the past participle of the main verb. For the regular -ir verb vieillir (to age), the past participle is vieilli.

How do I know to use cette before génération?

The word génération is feminine. A handy trick to remember is that almost all French nouns ending in -tion are feminine. Because it is feminine, you must use the demonstrative adjective cette (this/that) instead of the masculine ce.

What tense is changera and how is it formed?