Why do people who are older than me call me "sir" even after knowing my name?

Such is the socially acceptable manner of referring to a business associate, regardless of age (roughly on the same level as mr lastname). It is overridden by a degree of familiarity (ie amongst friends) or if you make it clear they should call you by your name. Youngins tend to dismiss social conduct as old people's trumpery. Some countries require less formality, whilst in places like japan or the army the norms are just shy of law.