Hi Gradgradinternational,
From what I was able to deduce during my interactions with alumni, current students, and the seminars programs hosted before the application stage, is that there is a mix of students and the average age is between 25 - 28. Most the programs would have around 50-60% of the class that is fresh out of undergrad and the rest is young or mid-career professionals either aiming to pivot or push ahead, leveraging the MPA/MA degrees.
I, for example, am in 30s and already have a Masters. Thus far, I have offers from Queen's (MPA), Waterloo (MPS), and Carleton (MPPA). I hope this helps.