Abu Layla’s inquiry regarding this question, and about one who recites the tashahhud while standing, forgetting, and not remembering until the bowing.

Abu Layla : There is a second scenario, Shaykh. Some of the elderly, from the bowing to the standing, naturally want to go down to prostration. So naturally, the imam says the takbir for prostration, but he is unable to prostrate and lift himself up. He is not yet in the process of going down, meaning he has not reached the ground. Has this happened?

The Shaykh : He follows the imam, even if slowly. Did you get the answer?
Abu Layla : Yes, Shaykh. I mean, if the imam, for example, prostrated and lifted his head, but he is not prostrating, does he have to prostrate?

The Shaykh : He follows, because he has no choice. If he prostrates and falls before the imam, he would be ahead of him, and here he is not obligated. If he lags behind him, he is excused. So he follows the action behind the imam. Yes.
Abu Layla : May Allah reward you with good.

The Shaykh : And beware of that.