How can we determine Splinter's age? Let's start with issue #1 of the comic and work our way backwards. For math purposes, we will treat the story as happening "now."
When the boys get back from their first mission above-ground, Splinter regales them with his origin. (You'd think he would have done that at some point before, but no.) He specifically says he's trained them for 13 years:
T -13
But he didn't begin training them the first day they were all mutated. "Within a year" they were fully grown, and it was still some time before they were intelligent enough to learn from him:
Which puts the initial exposure to the mutagen (and any longevity it might have bestowed) at T -14.
He'd also been living on the street for a while between losing his home and finding the turtles:
Our current estimate of T -15 is backed up by Shredder's own recollection of killing Hamato Yoshi:
Before he could take his revenge on Yoshi, Shredder needed to build up the Foot Clan in New York, a process which started when he was 18, and took a year:
T -16
So we know that Oroku Saki was assigned to America when he was 18, and we also know that he was 7 when his older brother died:
Those 11 years of training take us all the way back to T -27.
Even knowing the deadly fight between Hamato Yoshi and Oroku Nagi took place 27 years ago, Splinter had been Yoshi's pet for a non-zero amount of time before that. Splinter himself says he learned ninjitsu "slowly," suggesting it took substantial time even if he began doing it on the same day his new owner brought him home:
He may say his tale began "some 20 years ago," but it's definitely closer to 30. Splinter is at bare minimum 27, and somehow managed to live at least 13 years before being mutated, which would be twice as long as the (likely fake) Guinness World Record for "oldest rat" and the equivalent of a human living to be, like, 500 years old.
