The Marvel Cinematic Universe has a wonky timeline. There are certainly a few MCU plot holes, though we can determine the Avengers' ages. Marvel's creators relay the ages of certain heroes outright, while other ages can be determined by diving deep into the films for dialogue hints and context clues.
The MCU characters' ages range from surprisingly old to startlingly young. Moreover, they rarely align with the real-world ages of the actors behind the costumes.
For instance, although his depiction of Thor is undoubtedly convincing, Chris Hemsworth is nowhere near 1,500 years old!
Some promotional material pegs Nick Fury’s birth date as December 21, 1951. However, that may not count as true canon. It's clear Fury is considering retirement before the events of Captain Marvel, which takes place in the '90s. So, it wouldn’t be much of a stretch for Fury to be approaching 70 - actor Samuel L. Jackson’s real age - when he crumbles to dust at the end of Infinity War.
Hela's 5,000-year imprisonment results in some rather surprising minimum ages for various other characters. A flashback confirms Valkyrie, also known as Scrapper 142, battles Hela before her imprisonment. Consequently, Valkyrie must be at least 5,000 years old during the events of Thor: Ragnarok. However, time passes weirdly on the planet Sakaar, where Valkyrie chooses to live. And the warrior has Asgardian heritage, so it's hard to determine her exact age.
She's a member of an alien race, so Gamora’s age is hard to nail down. However, she seems to be in her late 20s when she's hurled off a cliff in Infinity War. Hers is an impressively young age for someone known as the deadliest woman in the galaxy. It’s also a departure from actor Zoe Saldana’s actual age.
Characters like the Grandmaster and his brother, the Collector, are known as the Elders of the Universe in the pages of Marvel Comics, and they’re each billions of years old. According to actor Jeff Goldblum, who portrayed the character in Thor: Ragnarok, the MCU follows comic canon. Goldblum revealed in an interview that the Grandmaster was born shortly after the Big Bang, making him just a hair under 14 billion years old.