Just where in the world are you getting this 8 year figure? The timeline isn't addressed in the books and Martin has refused to give any further details when asked directly. Just RAFO.
The wikipedia page, and that seems to be pretty accurate. The Hand after him merely lasted a year, and Jon Connington was 21 when he became Hand of the King. I think he would have specifically mentioned that Tywin quit being Hand as soon as his wife died and Tyrion was born. Plus we know for a concrete fact that he was still the Hand when Tywin asked Aerys for Cersei's hand in marriage, and Tyrion was around 4 then.
Its not all coming from ADWD either. I picked up the hints in my first read of AGOT. Dragon dreams, silver hair when all Lannisters have golden hair, Tywin's abrupt resignation and later betrayal. Aerys' fear of Tywin says a lot.
Being afraid of Tywin doesn't mean anything, since everyone in the Seven Kingdoms was afraid of Tywin Lannister. Aerys had fricked Tywin over enough without raping his wife, which led to a dwarf that killed his wife. Tywin would have quit being Hand immediately if his wife was raped by Aerys, because he wouldn't stand for a slap in the face like that one.
Plus Tyrion doesn't have silver hair, just slightly lighter hair than his siblings, who are twins and said to be absolute split images of one another to where some people had trouble telling them apart when they were kids.
And you do remember Tywin's dying words? "You're no son of mine."
Do you remember Tyrion's next phrase: "Now that's where you're wrong, Father. Why, I believe I'm you writ small." Not to mention the fact that people who actually know Tyrion well (aside from Jaime and Cersei) think that he was Tywin's true son. The theory is absolute bull shite.
This post was edited on 10/17 at 6:25 pm