Here Othello, having been manipulated into deep self-doubt by Iago, expresses the fear that he is too old for Desdemona. So, how old is Othello?
He says that he is "declined / Into the vale of years," and the Oxford English Dictionary says that the phrase "vale of years" means "the declining years of a person's life; old age." But Othello adds, "yet that's not far," so how far is "not far"?
If the "declining years" are considered to be those which come immediately after the halfway point of the journey from birth to death, then if "The days of our years are threescore years and ten" (Psalm 90:10), then "not far" may be around 35-40. However, in King Lear, Kent, who is 48, thinks that he must soon follow King Lear in death, so if Othello is also thinking that 50-ish is on death's doorstep then his "not far" may be younger than 35. On the third hand, maybe the "vale of life" starts later than the halfway point of 35.
On a related topic: How old is Desdemona?
I assume that she is not an old maid, but a woman who is of marriageable age. If that is so then she is no younger than 16, which is the age of Juliet in Romeus and Juliet, Shakespeare's source for Romeo and Juliet. In Romeus and Juliet 16-year-old Juliet is considered to be quite young, but in his play Shakespeare pushes her age back to 13 for dramatic effect. And Desdemona is probably not much older than 19, as 18 was, according to what I have read, the average real-life age for a woman to be married.