On Monday it was announced that Verstappen - son of former F1 driver Jos - will make history by becoming the youngest F1 participant of all time from 2015.
Not 17 until next month, Verstappen will replace Jean-Eric Vergne at Toro Rosso after a series of stunning performances in Formula Three.
With some questioning Verstappen's lack of experience, Alonso, who made his debut as a 19-year-old in 2001, believes the teenager should be allowed to prove himself.
"I think it is just one number on your passport, the age," he told Sky Sports.
"At the end of the day you need to be ready for the challenge and be ready for Formula One grands prix.
"Some people are ready at 17, some people are ready at 28 - that is what we don't know.
"So before saying anything we need to see how Verstappen does next year and after six to eight races we can see if he was ready or not. But at the moment anyone is ready."