I have in the past had goats get to 16 and 17 and neither of those seemed to lose their teeth, and kept weight on. Yet others get to their early teens, and lose condition. Its like any old anmal, coming into winter they do lose weight, and have to use any fat reserves to keep warm, so a coat may well be the answer.
As Wytsend says, the fast fibre is good, we use that for older ponies. Also soaked sugar beet, I soak it in warm water for the oldies, and they drink the water too. Oldies do struggle with certain types of hay, if this is the case, then one of the good quality horse chops are good.
Lack of minerals and vitamins can be the cause too.
To my mind, if the goat can still eat - may take a few tries to get feeds she can actually eat - and seems happy enough, then keep her going the best you can. Losing weight is part of the ageing process, and you may well find you cannot get the old goat fat.