How many years have the Tucks not aged?

The Tuck family has not aged for 87 years.  

At the beginning of chapter 7, the Tuck family narrates the origins of their immortality to Winnie Foster. They tell her they were heading east and looking for a place to live. Along the way, they came across the magic spring near Treegap. The family decides to make camp because the location is nice. All four members of the Tuck family drank from the spring. The horse also drank from the spring, but the cat did not. As the Tucks continued on living, they notice they no longer age. They also notice they don't die from things that should kill them. The spring granted them immortality, and it has been 87 years since the Tucks drank from the spring.