Aston Martin driver Alonso's first appearance for now-defunct Minardi was the Australian Grand Prix in Melbourne on March 4, 2001. Piastri was born in the same city on April 6 of that year.
While Piastri makes his debut, the race will be Alonso's record-extending 356th F1 start.
"Privileged to still drive here, better than ever. I have no concerns on age yet," Alonso told reporters at the Sakhir circuit with Piastri sitting in the same FIA press conference for the first time as a Formula One driver.
Piastri, who was reserve to Alonso at Renault-owned Alpine last season, returned the compliment.
"It's a privilege to race against Fernando obviously. I grew up watching him. My first go-kart ever was a Fernando Alonso kart, actually," he said.
"On paper, it sounds a little bit strange... for Fernando's career to be longer than I've been alive but it's a big privilege to race against someone like Fernando."
Alonso pointed out that Ferrari's Carlos Sainz (28), McLaren's Lando Norris (23) and former F1 racer Daniil Kvyat (28) had also started out with one of his karts.
Asked how many years he felt he had left to win a third title, Alonso said: "I don't know. Eight? 10?
When asked to "be honest", he replied: "five".
Piastri is one of three rookies this season, although AlphaTauri's Nyck de Vries (28) actually made his F1 debut at Monza last year as replacement for Alex Albon (26) at Williams.
The first race of the season is a tense time for all drivers, but all three played down any jitters ahead of their big weekend.
"I feel comfortable in the car. I feel like I've laid a good foundation and a base to work from so I'm relatively relaxed heading into tomorrow," said Williams' US newcomer Logan Sargeant (22).
(Reporting by Alan Baldwin, editing by Toby Davis)
