uninitialized member in code

Question

https://www.sololearn.com/compiler-playground/cLafwX2CrOKy
in this code i want to take input but no matter what i enter it says

Access denied - You must be at least 18 years old.
Age is: 0

./Playground/file0.cpp: In function 'int main()':
./Playground/file0.cpp:7:9: warning: 'a' is used uninitialized in this function [-Wuninitialized]
7 | int age = a;
| ^~~
age is 0 and this error message. why is this happening and how can i solve it