This depends on the function of the adjective. If it is qualifying the name (qualifier), then it must come after the name. E.g. el coche blanco means that, of all the cars, you are talking about the white one.
On the other hand, if you are using the adjective to remark some characteristic (epithete), it can go after or before the noun. In your example, it is clear which eyes you are referring to without the need for adjectives; you are talking about the eyes of the turtle. You only use the adjectives to remark some of their properties which may be important or interesting for what you are saying about them. In this case, the adjectives can be used before the noun.
Nevertheless, in Spanish is not so usual to put more than one adjective in front of a name. In this case, Los viejos ojos humorísticos would sound more natural, in my opinion.