1399 is probably pretty close to a real birth year, but can you provide better sources for this Clifford family?
The published genealogy link you gave doesn't work, and the personal tree link doesn't inspire confidence (it say his father was born in 1390).
If we make his birth date 1399 then we have problem of his father being born in 1424 and his son in 1504. Obviously there are many more problems here than just his birth date.
The dates I do know are that his wife's parents were married in 1401 and she was likely born around 1405, making the 1399 date reasonable but not proven by any real document.
William Clifford and Elizabeth Savage did not marry until 1406 at the earliest (her first husband died late 1405). So a 1407 guesstimate may be a better choice. He died in the lifetime of his father (d. 1438) leaving a son and heir born 30 November 1429. I can make that change but then he is supposedly the father of a Nicholas Clifford born 1524
Somewhere along the line we need to break it. There are so many wrong dates within three generations of your Lewis Clifford that it is going to take some real work to sort it out.
Some of the dates I gave come from Richardson and some from here.
Please add the tag Euroaristo to the post.