The US median age in metro areas is 34.9 years.
Phoenix has an average of 33.2. Of the group of cities that include LA, San Diego, Phoenix, Dallas, Houston and Atlanta, Phoenix has the oldest group, with others coming in as in the 32's. Rust belt cities like Buffalo and Rochester are in the upper 30's.
The ones I named are sunbelt cities, where living is attractive to younger, more mobile people. And, particularly, they are magnets for new immigrants. Phoenix is not one-third Hispanic, and we know that Hispanic families tend to be larger, swinging the balance on median age downwards with more children in households.
Scottsdale has a much higher median age as the costs there prevent it filling with younger entry level job holders. Older people have more of a "mall mentality". Millennials and immigrants don't.
Hartford's is 29.7, but with a heavy Hispanic skew. But Meriden, a city of 55,000 where the struggling mall is, also has a big Hispanic populatiion, but its median age is 36.7. And that's where the mall is. Go figure.
PS--I think I see the ever-frustrating now/not typo in your post. You mean Phoenix is NOW one-third Hispanic, right? (I have similar typo tics, typing "that" when I mean "than" and "thought" when I mean "though." Turns my prose into gibberish sometimes.)