The only thing which isn't natural is the repetition of 'years old'. It isn't necessary. If we know that you're talking about ages, there's no need to say 'years old'. Just the number is fine.
The sentence "I would never have said you're more than 26" sounds OK but, strictly speaking, the 'you're' should be 'you were'.
Other natural comments would be "I would never have thought you were more than 26" , "I wouldn't have put you at more than 26", "I'd have put you at 26 at the most".