Why replace a timing belt at 28000 mi. When there's no symptoms of a problem?

What is your question?

2007 Subaru impreza brought in to mechanic for a simple inspections by a 87 yr. Old lady. Although there was no symptoms of a problem with the timing belt (The car has 28000 mi. On it) it was replaced at a cost of over $500. She received no explanation. Sounds like a ripoff. What's your opinion?Thanks.