If Red started his journey as 10:
+3 +(5 to 10) + 2 = Red is 20 to 25 years old now.
If you want to consider the R/B manual, Red started as 11, he is 21 to 26 now.
If we consider what this anon said:
>>19118046(lel Gary)
It's very probably that Red is 20 or 21.