When it comes to age, you can't do anything when age is no longer by your side, particularly in football game which involves strength, agility, Mental energy, and lot more to play. Looking at his performance in this world, it's clearly seen that he has nothing much to offer in the field Play, and I don't really think that he suits the kind of striker that Chelsea needs right now.
However, buying him is still not that bad since he's on free transfer and there are other striker already in Chelsea.
If previously Chelsea could consider Aubameyang whose quality is not comparable to Ronaldo's, then I think Chelsea can also consider Ronaldo for at least 2 seasons. Short-term contract shouldn't be much of a problem for Chelsea and Ronaldo so I think he's still good for the Premier League especially for teams that need a striker.
But I don't know if Chelsea are interested in signing Ronaldo or not, but his attitude with Manchester United at the end of his contract seems to have been Chelsea's consideration before offering him a contract. So Ronaldo's current situation is really difficult, but he's still a striker worth relying on.