There are certain tennis strokes that adults can't learn. For instance,
adults can't learn to hit a "kick" serve that has a lot of topspin.
There is an absolutely fascinating video about plasticity of learning between adults and children that I'd highly recommend watching [1].
A man built a "backwards bicycle", where the handlebars steer the bike in the opposite direction to what you would expect. It was almost impossible for him to ride the bike more than a few feet at a time before falling off. Finally, after 8 months of consistent practice, he could ride it relatively well.
When he put his 6-year-old son on the bike (who'd been riding a normal bicycle for 3 years), he had it mastered just 2 weeks later.
Destin also admits
"It took me 8 months to learn how to do this, but I was only picking up the bike and running to the end of the driveway and back every day. I wasn't "ACTIVELY" trying to learn. Meaning... I wasn't struggling and trying to make my brain learn. I simply got on the bike every day, tried to operate it to the end of the driveway, turned around and tried to operate it back. The goal was to understand how my brain figured things out on its own, without trying to force it to. Many people have built bikes like this and figured it out in much less than 1 day by staying on the bike until they were able to master it. I had no timelines, and was using this as an exploratory activity to learn how I learn."
I'm interested in what would the difference be for them to alter a learned skill if they'd been practicing for the same amount of time (so that it's less set in stone)? I'm certain his son would have still been an order of magnitude or 2 faster, but probably quite a bit less than the original number.
I remember a few years ago I learned that I had been tying my shoes wrong my whole life (tying a granny knot instead), so my shoelaces would come loose at least once per day. It took me a few weeks of practice to manually think through the hand movements to make the new know before my brain got used to just doing it without thinking about it.
I'd imagine that as a child it should have been at least 10x easier to re-learn.
Perhaps not lose entirely, but certainly age can dramatically reduce the ability to learn certain things. This (rather long) article goes into detail:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2947444/
The section marked "Language Exhibits a “Critical Period” for Learning" is maybe the most relevant.
However, the article was written in 2010, and may be out of date. Understanding the brain's mechanisms and plasticity has advanced significantly over the last few years.