Give Satisfaction

The way I imagine them depends on what I'm reading.

If it's fanfic then I tend to picture them looking and speaking pretty much like their TV actors but with Jeeves being about 10 to 15 years older than Bertie.

When I read the books I imagine them in a different way but the age difference of 10 to 15 years stays the same here. I like to think that Bertie has an average or slightly below the average height and Jeeves is tall. Also that Jeeves has noticeably manlier (as in terms of looking mature) build than Bertie with broader shoulders but nothing muscle heavy or bulky and a bit of body hair in all the right places. Bertie is slender but not skinny and still has a more teenage boyish build and about the same dose of body hair as Jeeves. He will naturally get a more mature build with the years passing by but will always be a rather slim guy and narrower shouldered than Jeeves. I imagine them speaking more like how Martin Jarvis reads them on the audio books. I love how he reads Jeeves. To me his version comes closer to canon Jeeves than Stephen Fry's Jeeves if one chooses to rely on Bertie as a narrator when it comes to the description of Jeeves' emontionlessness. (I love Stephen Fry's Jeeves but his version requires accepting that Jeeves is able to express quite a lot of emotion with his micro expressions, his rather expressive eyes and his eyebrows move more than that famous fraction. ;) Martin Jarvis' Bertie may require a bit of getting used to but he makes him sound so much younger than Jeeves, which I think is great and he is good at expressing Bertie's cheerfulness and carefreeness. One can clearly hear the smile in his voice when he speaks.
I headcanon the stories taking place over the course of many years because it can be nice witnessing the age progression of someone you like and being able to look back on many years spent together.