import tkinter as tk

# Create Window
window = tk.Tk()

# Window properties
window.resizable(width=False, height=False)

## Widgets

# Postcode
postcodeText = tk.StringVar()
postcodeLabel = tk.Label(window, textvariable=postcodeText)
postcodeLabel.grid(row=0, column=0, sticky=tk.W)
postcodeText.set("Postcode:")

postcodeField = tk.Entry(window, bd = 5)
postcodeField.grid(row=0, column=1, sticky=tk.W)

# Radio options
var = tk.IntVar()
radioGP = tk.Radiobutton(window, text="GP", variable=var, value=1).grid(row=2, column=0, sticky=tk.W)
radioDentist = tk.Radiobutton(window, text="Dentist", variable=var, value=2).grid(row=3, column=0, sticky=tk.W)
radioOptician = tk.Radiobutton(window, text="Optician", variable=var, value=3).grid(row=4, column=0, sticky=tk.W)
radioSchool = tk.Radiobutton(window, text="School", variable=var, value=4, state='disabled')
radioSchool.grid(row=5, column=0, sticky=tk.W)

# Age
def ageChange(text):
    if int(text) <= 16:
        radioSchool.config(state='normal')
    else:
        radioSchool.config(state='disabled')
        var.set(99)

ageText = tk.StringVar()
ageLabel = tk.Label(window, textvariable=ageText)
ageLabel.grid(row=1, column=0, sticky=tk.W)
ageText.set("Age:")

ageFieldText = tk.StringVar()
ageFieldText.trace("w", lambda name, index, mode, ageFieldText=ageFieldText: ageChange(ageFieldText.get()))
ageField = tk.Entry(window, bd = 5, textvariable=ageFieldText)
ageField.grid(row=1, column=1, sticky=tk.W)

# Start window
window.mainloop()