19 (after timeskip)
After the timeskip
Following the two-year time skip, Luffy is 19 years old.
Second while luffy was poisoned by the jailor at impel down and ivankov treated him, ivankov told luffy the treatment would consume 10 yrs of his lifespan.
17 (Pre-Timeskip); 19 (Post-Timeskip)
Luffy. The captain of the Straw Hat Pirates is only 17 years old when One Piece begins, and in only two short years, he has toppled dozens of Marine officers, multiple Warlords of the Sea, and by the end of the Raid on Onigashima, even an Emperor of the Sea.
Ace is 3 years older than Luffy as evidenced by Oda revealing Ace's age through various sources. Since Ace was 10 years old 10 years ago, then Luffy was 7 years old back then.
In order to save the Straw Hat Pirate from the warden Magellan's venom, Emporio Ivankov used their Devil Fruit powers to alter the hormones in the boy's body to strengthen his immune system. The process was excruciating, lasted for hours, and ultimately took approximately 10 years off of Luffy's life expectancy.
Luffy dies from gear 5th strain on his heart and law gives up his life so they can win. This causes Luffy to go into gears without dying. While never said or confirmed explicitly, Gear Fourth also seems to have a similar effect. Luffy's body produces an incredible amount of steam when he uses the power.
Luffy's gears also put pressure on his body. Since using them is obviously more dangerous than smoking or drinking, Using gears repeatedly will eventually shorten his life to say 30 or 40. His 2 year training probably made him resistant to immediate side effects like weakness and becoming a dwarf after gear 3rd.
Though subtle, Luffy grows up and matures throughout One Piece while still retaining the essence of his character. Monkey D. Luffy would not be the same lovable character of One Piece without his childlike enthusiasm, his determination to be King of the Pirates, and his Gum-Gum Devil Fruit.
However, physical blows without Haki don't work on him due to his rubber body, and the only slashing-style ninjutsu Sakura is shown to know is a scalpel-like blade. With that being her only weapon to hurt Luffy with, it's likely she will lose this fight in embarrassing fashion.
In the Post-War arc, one of the Five Elders stated that "D." means danger. After the timeskip, Law mentioned the Will of D. once more when Doflamingo asked him why he had so much faith in Luffy. After visiting Laugh Tale, Kozuki Oden mentioned that he and the Roger Pirates had learned the meaning of the D. name.
Weaknesses. Despite these advantages Luffy is not immortal. Though immune to lightning, he can be damaged by other elements such as fire or ice, attacks involving cutting or stabbing and Haki-infused attacks.
Luffy has lost teeth before in a comedic fashion (Skypeia) and Usopp has lost teeth in battle (Alabasta, etc) and they have always come back.
So far it hasn't been confirmed nor canon if Luffy ever married Boa Hancock. And so far one piece hasn't finished the end yet. So until it's canon, Luffy won't be able to marry Hancock.
In the world, humans can live up to at least 141 years old, such as Kureha is, though her case was more likely due to her medical expertise to maintain such good health.
Luffy's possible timelines were revealed in an SBS for Volume 89. On a good timeline, he looks a little like Shanks at age 40 and a little like Garp at age 60; his love for meat only grows over time.
Boa Hancock is 31 in the story, twelve years older than Luffy's 19. On the other hand, Nami is just 20. It's difficult to gauge romances in anime by just using age difference, but Nami is closer to Luffy's age group and that is certainly a relevant point to consider in terms of their compatibility.
How old is Brook? Brook is by far the oldest member of the Straw Hat Pirates in One Piece. Technically speaking, this musician died when he was 38.
Ace promises Luffy that he will not die, but eventually fails to keep that promise at Marineford during the Whitebeard War. It is revealed that children born from noble families truly become nobles themselves at age 18, which leads to the reason why both Ace and Luffy departed and became pirates at age 17.