Let’s do a little math. Johnson was born on May 2, 1972. The Mummy Returns came out on May 4, 2001, when Johnson was two days over 29 — but we’re looking for his age when the movie was shot, not when it was released.
Films obviously don’t wrap and then release a few days later (though it would be pretty amazing if they did, especially with all the movies and shows being delayed because of COVID-19), so Johnson had to be younger than 29 during filming. According to IMDb, Sommers shot the film between May 1 and September 29, 2000. So, assuming Johnson was there from the very first day on set, he was 27, then turned and remained 28 for the duration of the production period. Final answer, then: Johnson is 28 years old in The Mummy Returns.
Even at that age, Johnson had the perfect build for action roles, which is no doubt part of the reason why his career took off so swiftly and surely — the other part being his undeniable charm. No matter how old he gets, Johnson will forever remain a cornerstone (cornerRock?) of Hollywood.