Built4speed, difficult/impossible to answer your question, as what has happened in recent years has been affected by the change in retirement age. I don't think there is such a thing as a 'typical' career path in BA. When things start to move on from the current stagnation, there will be shiny new types to fly, and it doesn't follow that the most senior wish to fly them.
Foe example, the A380 will be introduced in BA under our 'small fleet rules' where everyone is rostered (ie no/little choice). This, coupled with the lack of variety in routes, and also Airbus v Boeing considerations (people may not wish to undertake conversion courses towards the end of their career) means that it is a difficult call to make as to how senior/junior a such fleet may be.
BA offers a fantastic variety, expansion is ahead of us, my only advice is to get in as soon as possible and bid for whatever you want to fly once you are unfrozen, good luck.