date of birth unknown. Principal actor. There seems to be agreement on his age to have been 10 in 1594. A proper justification would have been nice though : there is nothing on the internet but his absence from the SDS-plot. A source that accounts for all the eleven-ups in this presentation.
Internet Shakespeare Editions places him in the 1594 company as a co-founder and child actor. Again, nothing there to substantiate the claim. But if he indeed was, than most likely as either the senior treble, or the leading lady. As the senior treble he would have been a boy between fourteen and sixteen (*ca. 1579) whose voice was about to break. Gilbourne was before James Sands apprenticed to Augustine Phillips, which dates his birth earlier, and allows to estimate ca. 1579 again.
However, Chr. Beeston seems to have been apprenticed to Phillips as well. It makes sense to assume that Phillips admitted apprentices at regular intervals, and to place Gilbourne in age halfway between Beeston (*1579-80) and Sands (*1584-88), That would result in 1582-84 as a fairly accurate guess. In the end, his absence from the SDS-plot tips the scales in favour of not yet being eleven. Pending a good reason to regret the decision : as the co-founding child actor of fifteen he is the perfect cast for a part that now has to be assigned to Robert Pallant. Young Robert, in his turn, seems better casted for a part that now… etc.