Yes, abacus training is beneficial for all age groups, including adults and elders.
Children (especially between ages 4–17) may learn faster because their brains are more adaptable during early development. However, adults can also successfully learn and retain new skills through consistent practice. In fact, learning abacus as an adult offers additional benefits such as:

• • Improved concentration and memory
  • Better mental calculation speed
  • Enhanced brain coordination and activity
  • A great mental exercise to keep the brain active

Like any skill, regular practice helps retain and strengthen what is learned—at any age.