Yahiko, during the end of the manga series. Meaning, he's grown up, already 15 years old, and now has Kenshin's Sakabatou, and has mastered the Kamiya Kasshin style, and far surpassed Kaoru. Yahiko was able to beat one of Sishio's remaining captain, that led the attack on Kyoto. He's probably no push over.
Okina is old and is blind on one eye. Yet he was able to put up a good fight with Aoshi.
Would Yahiko be able to beat Okina? The same Okina also at the end of the manga series. Fight to the death.