Why could Aisa use observation haki from birth? During the fight with Enel on Sky Island, the four priests, including Enel used mantra. Later on, we learn that Mantra is actually Haki.

Luffy trained with Rayleigh to learn Haki. While Luffy was born with it, he wasn't able to use it perfectly. However, at sky island there was a kid called Aisa who used Haki to to sense people from distance. She wasn't able to perform Conqueror's Haki since she was a child, but she was able to use Observation Haki since birth. How is this possible when even Luffy couldn't do that?
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...