Why is Ibn Abbas accepted to have been 13 years old at the time of the death of the Prophet Muhammad ﷺ?

A hadith in Bukhari states that Ibn Abbas was 10 years old when the Prophet died: >وقال ابن عباس توفي رسول الله صلى الله عليه وسلم وأنا ابن عشر سنين وقد قرأت المحكم > > Ibn Abbas said, "Allah's Apostle died when I was a boy of ten years, and I had learnt the Muhkam (of the Qur'an). > > — [Bukhari](https://sunnah.com/bukhari:5035 ) However he is generally accepted to have been 13 years old. On what basis was the age of 13 determined? Isn't the information in Bukhari more reliable?