string to age

#include <iostream>

char toDigit(char c) {
    if(c == '1') return 1;
    if(c == '2') return 2;
    if(c == '3') return 3;
    if(c == '4') return 4;
    if(c == '5') return 5;
    if(c == '6') return 6;
    if(c == '7') return 7;
    if(c == '8') return 8;
    if(c == '9') return 9;
    if(c == '0') return 0;
    return -1;
}

int power(int n, int power) {
    if (power == 0) return 1;

    int res = 1;
    for(int i = 0; i < power - 1; i++)
        res = res * n;

    return res;
}

int parseInt(std::string s) {
    using namespace std;
    int num = 0;
    for(int i = 0; i < s.length(); i++) {
        if(toDigit(s[i]) == -1)
            return -1;
        num = num + toDigit(s[i]) * power(10, s.length() - i);
    }
    return num;
}

int main ( ) {
    using namespace std;
    cout << "Inserire N (0, 130): " << endl;
    string age;
    cin >> age;

    int output = parseInt(age);
    if (output == -1)
        cout << "Numero non valido" << endl;
    else
        if(output < 0 || output > 130)
            cout << "Eta fuori range" << endl;
        else
            cout << output << endl;

    return 0;
}