Ace’s novel volumes give an account on his invitation into the Shichibukai:
- Ace got elected and given an invitation to be a Shichibukai at Sabaody.
- He refused the invitation and got into a fight against a Vice Admiral whom he defeated.
- He unlocked Armament Haki during that fight.
By deduction, Ace accomplished this in his first year out as a Pirate, at the age of 17; this is the same age in which Sabo is also shown using Armament Haki and imo, the Manga makes them out to be parallels, 2 sides of the same coin which may be why Ace’s death triggers Sabo’s recollection of his past – apologies for digressing.
Long before Ace dies, he visits Shanks, fought with Jinbe & Whitebeard and has become a Whitebeard Pirates; at this point in time, he’s regarded as a Super-Rookie (who is a Pirates who gains a 100m+ bounty within a year of reaching Sabaody) by Shanks, so it’s implied that he’d accomplished all this notable accomplishments between the age of 17 and 18.
By the time he’s become an established rank within the Whitebeard crew (probably spent at least 2 years in the crew), he fights Blackbeard and Ace at this point in time is a seasoned New World Pirate. Blackbeard says it’s been a long time since Ace has been hit and this is a Pirate who already knows how to use Armament Haki and we can guess Observation (and let’s not forget that Ace already possessed Conqueror’s Haki, which he initially had no conscious control over. He first demonstrated the ability at 10 years of age).
Ace may appear weak, but we shouldn’t forget he fought 2 of the strongest characters in the series; strong guys look weak against the strongest. We shouldn’t forget that Ace was probably not in peak condition against Akainu as he’d been imprisoned, tortured, probably unfed for days and fighting against a monster. He also never really got the opportunity to fight Akainu intelligently, therefore, it’s difficult to gauge just how strong he is. Even after the time-skip, he’s still somewhat held in high regard:
- Chinjao regards Ace as the only one who could have become Pirate King 2 years after his death. (This is only qualitative)
- Correspondingly, the next person he sees as capable of becoming Pirate King is the victor that led to Marineford, Blackbeard.
While we can’t quantitatively ascertain or determine his strength, we should probably appreciate that he was able to defeat a Vice Admiral by the time he reached Sabaody which not even Luffy pulled off.