I can't help with birth or death.
But there were more children than listed on his profile. There were two births recorded at Rehoboth as children of John Myles Jr.: John b. 19 November 1666 and Mary b. 16 January 1669 (unknown if o.s. or n.s.). This makes sense because the compact that founded Swansea was not signed until February 1668/9 and lands were not distributed until the following year. So births after this founding were then recorded at Swansea, but before that, the land was part of Rehoboth.
These births are early enough that it is hard to imagine there were three John Myles's. So probably John Myles Jr. here was the son of the Rev. John Myles.
This is confirmed somewhat by a deed from 1693, in which John Myles of Boston, mariner, and his wife Sarah sold land at Swansea which is stated to have been willed to him my his grandfather, identified as John Miles Sr. This is interesting because I find no such will in my brief search, although this deed confirms that one existed.
This John had to have been born by 1672. So it seems to be the John recorded born at Swansea in 1666.
It seems that John was married by 1666, so the current 1651 as the terminus ante quem for his birth is too late. I'd guess you're looking for birth in Wales by 1645.
There is another deed recorded 14 March 1711/12, in which apparently John Miles conveyed land at Rehoboth to Peter Hunt:
Unfortunately, that deed is in the portion of Vol. 7 of Bristol Co. deeds that was not filmed with the regular filming by the LDS church. They film they refer you to can only be viewed at an FHC or affiliate library. This could be either John the son or John the grandson, I would think.
John the grandson died around 1710, intestate, and letters of administration were granted to his widow Sarah on 13 October 1710 in Boston.