If he'd said something before R1, it would have been embarrassing for him to lose to that kid, and embarrassing for the kid to be the one to have ended Roddick's career (probably only to lose in the next match). The kid is American so Roddick doesn't want to undermine him.
He probably would have announced it in his post-match press conference and the USTA would've arranged some time before one of the 7 pm matches later in the tournament for Roddick to come out and wave goodbye to the crowd (Remember how Sampras came out?).
What is it with MTF and members calling anyone under 25 a kid? Williams is 21, he's no child. Roddick was lifting the US Open trophy on his 21st birthday. No need to emphasise his age constantly and unreasonably at that. In 10 years, MTF members will be proclaiming 30 year men as 'kids' because the top 50 will be occupied by those receiving their old-age pension.