In this usage they are interchangeable. Elder (meaning older) is used for describing people, with very few exceptions (I can think of only one). Older can be used on anything. Elder can only be a noun or an attribute adjective. Older is more versatile.
If English is not one's first language, I'd recommend always using 'older'. It sounds like you have it down though.
Because the adjective in singular form is 'mayor' and it ends with a consonant. So, the plural form will be 'mayores'. The rule is basically that: Adjectives that end in -o have four forms: alto, alta, altos, altas. Adjectives that end in -e have two forms: inteligente, inteligentes. Most adjectives that end in a consonant have two forms: popular, populares (form plural by adding -es).