Can a piano be tuned after 20 years

Yes! Pianos can be tuned after a long period of not being tuned.
I regularly tune pianos that were not tuned for 10, 20, 30 years, and tune them to the standard, which is A440 concert pitch.

One piano I recently tuned had not been tuned in 60 years!

A piano that has not been tuned in a while, takes longer to tune. We charge a flat rate, regardless of how long it takes to tune, just $300.

Leaving the piano untuned for a long time is not recommended. The piano will sound better, and hold its value better if it is regularly maintained.

A piano that has not been maintained for a while, may also need other repairs or adjustments. The parts inside the piano can wear and need regulation (adjustment to work properly). Even if the piano is not used for 20 years, parts inside the piano can be affected by moisture in the air, as well as can compress over time. So your piano may need additional follow up visits, to help bring the piano up to its full potential.

Before tuning an older piano that has not been serviced in a while, I will do an inspection. I am both looking and listening to the piano.
There are times when it can be tuned, but not to the same level as a modern piano, but I will still be able to tune it and bring out the best. It should be noted, that as pianos get older, there is an increased risk of strings snapping.