Was bushido an ancient code, or invented later?

It was both, and neither. Bushido, "the way of the warrior", is described as a samurai moral code on attitudes, behaviour and lifestyle. Its origins date back to the Kamakura period, but the code was formalized in the Edo period. There was no single ancient rulebook, though. Historically the samurai followed multiple warrior codes, which varied by clan, by person and by era.

The word itself is surprisingly late. The term bushido is first attested in a 1616 chronicle of the Takeda clan, and it is "rarely attested in pre-modern literature", although a code of honor did exist among writers and historians. And the ideals were not always practised. In Kamakura-period fights, warriors often disregarded the honorable norms, and announcing your lineage was mostly a way to brag. When Mongols faced warriors who tried to announce their lineages, they usually cut straight through them. Two and a half centuries of Edo peace then refined bushido from a focus on valor in battle toward moral integrity.

The version most of the world knows came later still. The term entered common international use through Nitobe Inazō's 1899 book Bushido: The Soul of Japan, read by many influential Westerners. Wikipedia's article on bushido calls Nitobe's version a romanticized interpretation that differs from other historical literature by the samurai. Some researchers say that chivalric bushido was invented in the 19th century; others reply that plenty of earlier literature on warrior codes exists. Both sides have a point.

So bushido is real history with a rewritten label: old customs, a 17th-century name, an Edo-period refinement and a Meiji-era polish.