It's old enough to top, question is why? Mostly I top that young when the seedling is scrawny or has too much distance between leaves (aka "inter-nodal distance") . Your plant looks vigorous and promises a bushy shape. Why would you top that? Only reason I see to top is to wait until the plant is 6 weeks in dirt, then take the top 6-8 inches as a crown clone. Any topping before mom is 6 weeks will probably not root as a clone. So, I would wait to take a killer clone.
Good luck, BigSteve.