Products like Mederma Scar Gel claim that they are "clinically proven to improve the softness, texture and overall appearance of scars." Mederma in particular promises to improve both old and new scars.
Wikipedia's entry on scars notes some treatment options:
Ointments and semiocclusive dressings including mineral oil, lotions, and petrolatum-based ointments are recommended under guidelines as they promote moist healing. One 1996 trial found healing was improved but physical characteristics were unchanged, while a 2000 trial of a triple-antibiotic ointment found reduced scarring.
Unfortunately, I cannot read the cited 2000 results without some sort of paid subscription. But the comments that these gels work by "promoting moist healing" implies that they are only effective when the scar is healing, and therefore wouldn't work on old scars.
I am doubtful that scar gels show appreciable improvement in the appearance of scars, especially old ones. Is there any conclusive evidence that would cause me to believe otherwise?