Too Young?

Just out of curiosity...why is it rude to tell someone that they look old, but not that they look young?  Personally I feel that commenting or speculating on someone's age is ALWAYS rude. Telling someone how young they look is not a compliment, it is patronizing. 

To preface my present irritation, I got carded at the grocery store for buying cough medicine.  No, not carded, interrogated.  I have been fairly sick for four days (stupid spring colds!) and yesterday I drank the last of my NyQuil sometime between calling in sick for work and spending a restless night coughing and wheezing.  Tonight I managed to drag myself to the store to buy some cough medicine.  To give the store the benefit of the doubt, perhaps my pajamas, rheumy eyes, hoarse voice and wracking cough contributed to the overall appearance of someone up to no good.  OBVIOUSLY I must be an at-risk teenager purchasing NyQuil to fuel my dextromorphan addiction.

The extent of my conversation with the cashier:
Her: "What is your birthday?"
Me: "July 22, 1983"
Her: (looking at me critically over the top of her glasses) "Really?  Let me see your ID."
Me: (cough, hack, wheeze) "Are you kidding?"
Her: "No.  You need to be 18 for me to sell you this product."

I understand why I am carded at the bar or liquor store, although I will most likely be well into middle age before I have the appearance of a 21 year old.  But an obviously sick person with orange juice, Kleenex and NyQuil is not trying to pull one over the King Sooper's cashier!  Toxic waste flavored, poison-colored NyQuil is NOT something I look forward to ingesting and if I was going to be falsifying my age, it would be for something better than cough medicine.  Oh wait, now I don't actually have to prove my age for anything until I claim Medicare!

Tonight's adventure at the store is just another entry on the interesting list of incidences where I have been mistaken for someone much younger.  Among the better stories in the recent past...

1. Signing up for a gym membership: "Are you older than 18?  Otherwise, your parents need to sign the contract." 
2. On the plane from Paris to Washington, DC, while sitting between a middle aged couple: "Did you and your parents have fun in France?"
3. While getting a flu shot: "You have to be 18 to get a shot, otherwise you need your parent's permission."
4. While attending an adult ed class held at a junior high during their back-to-school night: "Did you pick up your schedule?  When you get it, you and your parents can go to the auditorium."

I might appreciate this some day, but right now I would just appreciate some respect.  Or at least cough medicine that tastes less like poison...