I can think of two. The first was at Stirling Bridge, in 1297 when the English outnumbered the Scots some 4 to 1, but the 72-year old English commander, John Warrenne, Earl of Surrey was not keen on giving battle in swampy land where the Scots were "at home." But the 40-year old Hugh de Cressingham led half the army (the part under his control) across Stirling Bridge against orders. When half of this force was across the bridge, the Scots attacked from ambush at the odds of 1- to 1, crushed both halves of Cressingham's force, and even carried the battle successfully to Warrenne's remaining troops.
The second was the Waterloo campaign, after the defeat of the Prussians by Napoleon at Ligny. There, the badly wounded 72-year-old Marshal Blucher insisted on leading the remnants of his defeated army to Waterloo to reinforce Wellington, over the objections of the much-younger Gneisenau. The Prussians made it to Waterloo on time with sufficient reinforcements, and the rest is history.
Are there other instances where military commanders of very different ages were of the opposite opinion? What were the results?