I don’t see the need for complication. Vehicle replacement is about vehicle not lines. A button on the vehicle called Replace now is enough. Magically reset the vehicle age, running cost and value.
Usually in lines there are vehicles with extremely different ages.
Also I don’t really replace vehicles when the game tells me to. they are profitable for much longer.
The button can also be right in the vehicles window. Just sort by age and click replace next to the vehicle. Very easy.
And most important: REMOVE POP UP ABOUT VEHICLE AGE. That is very annoying and loud and unnecessary. The player should check the age of his fleet from time to time in the vehicle window.