View is not updated using @State property wrapper in SwiftUI

Question

The view does not get updated with using @State wrapper. The view gets updated without using @State wrapper but has to do it manually.

import SwiftUI

struct SwiftUIView: View {
    @ObservedObject var people: People
    var body: some View {
        VStack {
            ForEach(people.peoples){ person in
                HStack{
                    Text("\(person.age)")
                    if person.ismale {
                        Text("Male")
                    } else {
                        Text("Female")
                    }
                    Button(action:{ person.age += 1}){
                        Text("Age+1")
                    }
                }
            }
        }
    }
}

struct Person: Identifiable{
    var id = UUID()
    @State var age: Int
    var ismale = true
}

class People: ObservableObject {
    @Published var peoples: [Person]
    init() {
        self.peoples = [
            Person(age: 10),
            Person(age: 12, ismale: false)
        ]
    }
}

struct SwiftUIView_Previews: PreviewProvider {
    static var previews: some View {
        SwiftUIView(people: People())
    }
}

For some reason, when the button is clicked there are no changes to the view.

If I change it to

Button(action:{ **self.people.peoples[0].age += 1** }){
       Text("Age+1")
}
...

struct Person: Identifiable{
    var id = UUID()
    **var age: Int**
    var ismale = true
}

It updates, but the button would not be dynamic. However, if I do person.age += 1 inside the ForEach loop without the @State wrapper for variable age.

ERROR: "Left side of mutating operator isn't mutable: 'person' is a 'let' constant"

I thought I understand what does Struct, Class, @State mean... I guess not... Thank ahead for any solutions.