They do sleep more, but should still eat the same. Any sign of loss of appetite, loss of fur or pain/stiffness needs to seen by a vet and treated accordingly. Some older guinea pigs can lose some weight (condition) slowly in the last months of their life, or never regain it after an illness/operation despite having a good appetite.
Our first family guinea pig lived to the ripe old age of nearly 10 years, back in the seventies. Nowadays, many piggies only live to 4-6 years, due to indiscriminate breeding. There are exceptions, though!