It would be better than a period since seven years old already isn't a sentence. I would use a comma. Or, if the dialogue is intended to be exclamatory… Wow, look at my boy!
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Snarf"I'm proud of you, son" he said. "Wow, look at my boy. Seven years old already. Where does the time go?”Would it be better if there was a colon after boy, so that it read, "Wow, look at my boy: seven years old already. Where does the time go?"It's good with the period. People often do not speak in complete sentences, the meaning is clear, and the cadence
SnarfWould it be better if there was a colon after boyIt would not be better. It would be worse.
4444mvWhy not a semicolon?Because you don't have two independent clauses and you don't have a list.