As others have already said in this thread, the market value that transfermarket is defining is not really a hard number that is valid for everyone that is involved in football. It is just a value that this site is setting following their own rules and parameters. Other sites are doing the same and may have different values for each player.
That being said, it is pretty obvious that the market value of Cristiano Ronaldo will slowly but surely become lower and lower because he is already 37 years old. I would say 20 M€ for a player of his age is still an excellent value. Many good players never had such a high value even in their prime.