The oldest military unit still existing is the Regimiento Inmemorial del Rey número 1 (King´s Inmemorial Regiment)... the unit is active from 1248 to today (770 years) It was stablished by the King Ferdinand III of Castille and Lion and had fought from the conquest of Seville to Afghanistan...one unit fought in every continent. Its nickname is El Freno (The Brake)...because it was stablished to "stop" (brake) the enemies of the Crown.
Between battles regiment fought: Seville, Navas de Tolosa, Salado...Inmemorial fought Coldstream Guard in battle of Brion in 1800 and of course America, Asia etc.
You are right. Regiment didn´t fight in Navas de Tolosa... beccause it was stablished 36 year later. The unit stands from 1248 (and took part in the conquest of Seville in 1248). But it fought in battle of Salado in 1340 etc etc.
Between 1931 to 1976 the Regiment was named Inmemorial Nº 1 but recovered the former name in 1976 (King´s Inmemorial Regiment). Because the defense of Rome in 1849, the regiment was honoured by Pio IX with the Pope´s blessing that is engraved in the flag.
King´s Inmemorial Regiment war Flag:
The colour purple means Castilla and was granted by the King in the year 1248 opposite Seville. The differnt shields you can see are territories belonged to the Crown and the regiment fought there.
The coat of arms:
El Freno is the nicknamed from 17th Century granted by the King Philip IV on September 10th, 1632. It was named the Brake because "braked" the King´s enemies.
In this regiment, 4 men were awarded with the Laureate Cross of Saint Ferdinand (the highest Spanish military condecoration for gallantry) but the Laureate only exists from August 31th 1811. Only it is possible to be granted in war by action against enemy. The Laureate men in this regiment were in Morocco in 1859, in Cuba in 1895 and in Civil War in 1937 (2 men).
The Royal Tank Regiment can trace it's history to 1916. So it is a new boy on the block. It is however, the "oldest" Tank Regiment in the world. Older Cavalry Regiments may now operate on tanks, but they remain "cavalry" Regiments, not Tank Regiments.