Rebbe Rashab was in his 20th when his father was Nistalek. His older brother was the Razo.
Around 11 years after his father's passing he moved his seat to his father. The Rashab was in his 30th then.
The Rebbe however counts the first mamer recited by the Rashab “Keser Yitnu lecha” right after the Shiva, as the beginning of the Rashabs leadership.