Questions & Answers about Оба брата на два года старше Ивана.
Why do we use на два года here?
This is the comparative of difference. When you want to say how much more or less something is (like "two years older"), you use the preposition на followed by the accusative case for the quantity (два года).
Why is Ivan in the genitive case (Ивана)?
In Russian, there are two ways to say "than someone". You can use чем + nominative (чем Иван), or you can simply drop the чем and put the person being compared into the genitive case right after the comparative adjective, which is what happens here (старше Ивана).
Why is it Оба брата and not Оба братья?
The word for "both" (оба) acts just like the number two (два). For masculine and neuter nouns, it requires the noun to be in the genitive singular, not the plural. So we use the genitive singular брата instead of the nominative plural братья.
Why is the word for "older" старше instead of старее?
While старее exists, it is mostly used for inanimate objects to mean "more old" or "more worn out." When talking about the age of people, you should always use the irregular comparative form старше.