My dad is thinking of going there and wants to find a job but doesn't guarantee one, he already has the visa but he doesn't want to risk going there without finding the job. I heard they prefer young people. My dad is about 45. He doesn't speak danish. So is it possible to find a job there?

What sort of skills does he have? That will make a difference whether Danish skills are required or not. If he's a doctor he needs to speak Danish not only to work with patients but also to read and write up medical reports. If he's an IT professional he can probably get by in English as most big tech companies employ staff from all over and the working language is English. A friend if mine worked for Microsoft in Denmark about 7 years ago and they spoke English at work since they had over 40 different nationalities working in her office

How does he have a visa if he doesn't have a job already lined up?

Depending on his nationality, he either doesn't need a visa, or can't have a visa without having a job to go to. If he is having second thoughts about the job that is tied to the visa, he can't just change his mind and take some other job - that invalidates the visa.

Unless he is incredibly highly educated and in demand in Denmark, or lines a job up beforehand, then he would find it extremely difficult to find work as a foreigner who doesn't speak the language. Foreigners are dishwashers and toilet scrubbers without language and a high education here.