Group Array of Arrays by parameter

ArrayExtension.swift

extension Array {
    
    func groupBy<G: Hashable>(groupClosure: (Element) -> G) -> [[Element]] {
        var groups = [[Element]]()
        
        for element in self {
            let key = groupClosure(element)
            var active = Int()
            var isNewGroup = true
            var array = [Element]()
            
            for (index, group) in groups.enumerate() {
                let firstKey = groupClosure(group[0])
                if firstKey == key {
                    array = group
                    active = index
                    isNewGroup = false
                    break
                }
            }
            
            array.append(element)
            
            if isNewGroup {
                groups.append(array)
            } else {
                groups.removeAtIndex(active)
                groups.insert(array, atIndex: active)
            }
        }
        
        return groups
    }
}

//Usage 
struct User { var age = 0 }
let users: [User] = [User(age: 2), User(age: 4), User(age: 5), User(age: 5), User(age: 2)]
let groupedUser = users.groupBy { $0.age }
print(groupedUser)

It's work perfect with struct but not woking with array of custom class
Any solution?