I was definitely thinking of My Side of the Mountain during the session when we discussed Lúthien's treehouse!
The house where I grew up has a pair of old sycamore trees on the property. The trees are certainly over 100 feet tall. I doubt they were more than 200 years old, but I couldn't say their actual age. One did have a hollow space in the bottom of the trunk, large enough for a dog to crawl into.
(The pair of sycamore trees on the right are only 20-25 years old, sprouting up after the pond was added. The spring house to the left of the old trees had been there for...a very long time. The property was inhabited since the 1790's. This photo was taken in 2012.)
So, maybe give it another 400 years, and....
To me, what is important is that the treehouse make people think of magic and nature and live growth, rather than something...built. So, not a flet.