The only reason why people get offended by being referred as old is because they're not getting wiser with age.
Seriously. People. What's really up with you? What, just what, is the exact thing that upsets you so much when somebody innocently / unintentionally / inadvertently / even deliberately (circle appropriate choice) refers to your chronological seniority regardless of implicit or explicit manner. What in your entire life of upbringing and social education taught you that being "old" is a bad sinful shameful and criminal activity?
Understand this: there is absolutely nothing wrong about us growing older. We are in fact nearer to our grave with each brand new sunny day. And that is all natural. In fact, somebody growing younger would be undoubtedly out of this world and classified as "wrong" with all certainty. Nobody ever returns to his/her mother's womb and then proceeds to shrink and divide back into an egg/sperm pair. Nobody. That is not the purpose of our lives. Accept the truth right in front of you and stop denying yourselves the path of maturity towards wisdom and concern for things that truly matter in this world.
The sooner you realise the pursuit of "youth" is a fruitless activity illustrating your desire for your past naughty and foolish selves, the better. It is certainly not a desire to grow into a person of stature, spirit, wisdom, courage, knowledge, skills, and ultimately one who supports the next generation to greater heights. Focus on being a person others will look up to. Vainly attempting to look or act "young" earns you very little respect. Do not look backward! Only remember what has happened, learn as much as possible from those experiences, and spearhead forward to be a better person of the future.
And then you will see with extreme clarity, that age just becomes an absolute number with no relative value of "young" or "old". That simply becomes totally moot and an issue not worth pondering over.
To quote the comedian Bob Monkhouse: Growing old is mandatory. Growing up is optional.
Now excuse me while I change into a new pair of Depend.