- David Sheridan would have been born in 2262 and given the urn when he was 16 years old in 2278. That matches with what Delenn told the time-flashed Sheridan in the cell on Centauri Prime that "our son is safe".
More explicitly, David Sheridan would've been three in 2265, when Legends of the Rangers was set.
^That plus Martell's conspicuous lack of a headbone.
For whatever reason, JMS likes to use the name "David"...a lot and as such there's a sizeable number of them in the B5 canon. Similarly there's an awful lot of female characters named "Elizabeth" too.