We've always had a zoo membership because my husband and I love the zoo.
When our kids were babies, they mostly slept through it.
When they were toddlers, they ran through it.
When they were elementary school age, they loved the big cats, the elephants, and the snow cones...
In middle school, they just wanted to hang out with friends and walk ahead of or away from the parents.
Now our oldest is in high school. He usually wants to find a place to sit and text with his friends telling them how boring the zoo is and why wouldn't we let him stay home and this is so dumb and....
...while his middle schooler brother walks around with his friends hopping from one junk food kiosk to the next...
...while my hubby and I go around to our favorite exhibits enjoying ourselves.
So... I'd say like... maybe kindergarten through third grade is prolly like the best age for taking your kids to the zoo.