be = 是 get = 成 The difference is not even subtle. It's quite considerable.

I think it depends on the context of the sentence. You might use "to be older" when you are comparing two or more things. For example, "to be older than another person." When you say get older, that is usually used in a sentence such as "Every birthday I get older." Or another example "I am getting older every year." When you use "be older" you can also use another form of "be" in commonly used sentences such as "he 'is' older than me." Instead of "be" I used "is"