We are told that Lord Yu is the oldest of the System Lords, and eventually he becomes unable to take a new host, and the sarcophagus' healing effects are greatly dimished. By season 8, he is almost completely senile, unable to even speak for himself at the meeting of the System Lords and the Tauri. Assuming Apophis had only one host, he was around 5000 years old, so obviously Yu must be much older than that.
He has been referred to as two different figures: "Yu the Great" and the "Jade Emperor." He could have assumed both of these roles at some point in his life; the persona of Yu the Great would put his age at around 4200 or so (Yu the Great was born roughly 2200 BC). This couldn't be his oldest persona, as that would mean Apophis was older than him.
However, his persona of the Jade Emperor seems illogical; according to myth, it took Yu Huang Shangdi 1750 kalpas (each kalpa being 129,600 years) plus another 100,000,000 years (for a total of 226,800,000 years) to become the Jade Emperor. We have seen many times in the past that historical or mythological figures were actually Goa'uld, however in this case that simply can't be possible... right? I mean, as far as I'm aware that would mean his existence would have possibly predated that of the Ancients. So, how old does a Goa'uld have to be to become senile and experience the other adverse effects of time that Lord Yu was suffering?