Albert Barnes Commentary 1 Chronicles 22:5

"And David said, Solomon my son is young and tender, and the house that is to be builded for Jehovah must be exceeding magnificent, of fame and of glory throughout all countries: I will therefore make preparation for it. So David prepared abundantly before his death." — 1 Chronicles 22:5 (ASV)

Young and tender - The exact age of Solomon at this time is uncertain, but it cannot have been more than 24 or 25. It may have been as little as 14 or 15. Compare the note on 1 Kings 2:2.