A little theory that has always been my head canon.

1) Before Celestia and Luna there were no alicorns. 

2) Celestia and Luna were created to counter Discord. Discord is the god of chaos so maybe the ponies then were unable to stop him. Celestia and Luna's "parents" were likely unicorns who somehow manage to "magic" them into being. (probably some magic related to the sun and the moon) This explains their ageless-ness and all their differences from other ponies.(flowing mane, greater magic than usual etc)

3) Eventually, Celestia and Luna were hailed as  rulers. Equestria is the main place they rule over but there is no telling if they have vassal states. (since we don;t know IF Crystal Empire is a vassal state to Equsetria or not) Anyways within their realm, after some time, royal status, or in this case being a Princess= Being an Alicorn. And it is likely that as the highest power in the realm, Celestia and Luna conferred royal titles on other ponies by turning them into alicorns. And this is probably what happened to both Cadance and Twilight.  

This also explains why Cadance appears to age normally and does not have Celestia and Luna's flowy manes. It is likely that when Crystal Empire fall under Sombra's curse, some of the royal family managed to escape and lived under Celestia's and Luna's protection since then, waiting for the day when they can reclaim their Empire. Cadance is probably from that lineage and was conferred the title of Princess at birth and hence made an Alicorn.

4) Any kind of relations that Celestia and Luna have were related to their parents (the unicorns who somehow resulted in their existence) instead of to the princesses themselves. This includes Prince Blueblood and Cadance.

5) Flowing manes. As can be seen from Luna's case, it is likely that Celestia and Luna get flowly manes once they manage to realise their full potentials or certain extent of their potentials.This is (till the shows indicate otherwise) probably unique to Celestia and Luna only due to them not being regular mortal ponies.