MainActivity.kt

class MainActivity : AppCompatActivity() {
    
    private val userAge by sharedPreference(this, -1)
}

SharedPreference.kt

typealias sharedPreference<T> = SharedPreference<T>

class SharedPreference<T>(
    private val context: Context,
    private val defaultValue: T) {

    private val preferences: SharedPreferences by lazy {
        context.getSharedPreferences(context.getString(R.string.app_name), Context.MODE_PRIVATE)
    }

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
        return findPreferences(property.name, defaultValue)
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        savePreference(property.name, value)
    }

    @Suppress("UNCHECKED_CAST")
    private fun findPreferences(key: String, defaultValue: T): T {
        with(preferences)
        {
            val result: Any = when (defaultValue) {
                is Boolean -> getBoolean(key, defaultValue)
                is Int -> getInt(key, defaultValue)
                is Long -> getLong(key, defaultValue)
                is Float -> getFloat(key, defaultValue)
                is String -> getString(key, defaultValue)
                else -> throw IllegalArgumentException()
            }
            return result as T
        }
    }

    @SuppressLint("CommitPrefEdits")
    private fun savePreference(key: String, value: T) {
        with(preferences.edit())
        {
            when (value) {
                is Boolean -> putBoolean(key, value)
                is Int -> putInt(key, value)
                is Long -> putLong(key, value)
                is Float -> putFloat(key, value)
                is String -> putString(key, value)
                else -> throw IllegalArgumentException()
            }.apply()
        }
    }
}