Why could Aisa use observation haki from birth?
EVERYONE can train perception and armour haki, it's not a special gift. Only the emperor haki is extremely rare and only one in a million can use it (cf. http://onepiece.wikia.com/wiki/Haki ).

Quite clearly some people are more gifted than others, and Aisa just happened to be extraordinarily gifted with her perception haki, as nobody trained her. She naturally mastered it, and at such a young age it's a big feat but not impossible. On the other hand, Luffy had to be trained by Raleigh himself and he most probably still isn't as good as her with his perception haki. But his personality goes better with armour and emperor haki so he specialised in those!

Similarly, to get better at sniping Usopp needs to learn perception haki, but he will probably stay weak in armour haki, while Sanji should get armour haki on his legs if he wants to be competitive...