In David's old age, which of his sons declared, 'I will be king!'?

Answer: Adonijah

Adonijah son of Haggith exalted himself, saying, 'I will be king!', and gathered chariots, horsemen and fifty runners.

“At that time Adonijah, David’s son by Haggith, began to exalt himself, saying, “I will be king!” And he acquired chariots and horsemen and fifty men to run ahead of him.”— 1 Kings 1:5 (BSB)