John Gill Commentary 1 Chronicles 23:1

"Now David was old and full of days; and he made Solomon his son king over Israel." — 1 Chronicles 23:1 (ASV)

So when David was old and full of days
Perhaps was now in the last year of his age, about seventy years old, though before he was bedridden; see (1 Chronicles 28:2) ,

he made Solomon his son king over Israel ;
declared him to be his successor; this was before the affair of Adonijah, for then he ordered him to be anointed king, and placed on the throne; and this aggravated the rebellion of Adonijah, that it was against the declared and known will of his father.