Question

when I am (old) ,I will join taekwondo classes​

Answer 1

Answer:

when i will be old, i will join taekwondo classes.

Answer 2

Answer:

Answer would be like this :-

When I'll be Old, I will join Taekwondo classes.

but meaning is

जब मैं बूढ़ा हो जाऊंगा, मैं तायक्वोंडो कक्षाओं में शामिल हो जाऊंगा।

You should us it as

when I'll be grown up, I will join Taekwondo classes

meaning is

जब मैं बड़ा हो जाऊंगा, मैं तायक्वोंडो कक्षाओं में शामिल हो जाऊंगा

Hope it's help