Learn Croatian: Moja baka ima sedamdeset godina. - My grandmother is seventy years old.

It is simply a combination of the base number and ten. Here, it is sedam (seven) plus deset (ten) combined into one word. You will see this exact same pattern for fifty, sixty, eighty, and ninety!

Why does the sentence use ima (has) instead of the verb "to be"?

In Croatian, you do not say that someone is a certain age. Instead, you say they have a certain number of years. Since baka (grandmother) is "she", we use ima (she has) from the verb imati (to have).

Why does the word godina (year) look like it is in the singular form here?

It is actually in the plural! In Croatian, numbers from 5 upwards (including 70) require the noun that follows to be in the genitive plural case. By coincidence, the genitive plural form of godina is exactly the same as its base singular form: godina. If the grandmother were 72, the rule changes and you would use the genitive singular form: sedamdeset dvije godine.