From about 12 or 13 my DC have had an inch of Champagne in the bottom of a glass on Christmas Day and New Year's Eve to join in the social ritual of toasting and celebrating with family and friends. Nothing else at any other time though. Neither of them enjoyed the taste when they were this age and I would find an abandoned glass with the Champagne barely touched but they wanted to be part of the moment.
Similarly, my youngest really doesn't like tea but I've noticed that if a family friend or my SIL pops in and I make a pot of tea and put some nice cake or biscuits out, she'll ask for a cup too. Again, I think it's about being included in something warm and sociable and wanting to not to be excluded from a social banding ritual.
With our eldest, we let her and her friends have one (weak) cocktail and a glass of wine each with her birthday celebration dinner at home from the age of sixteen and her friends' parents have done similar things.
We've let her take two alcopops with her to a teenage party since she was sixteen too, because I'd rather her drink them and know what she's had rather than taking swigs of other peoples stuff and not know how many units she's had.
She's now 17. If she and her boyfriend have supper at home with us at a weekend, he usually has a couple of small beers and she might have one or two small glasses of wine. They're great company and we can still be found sitting at the table long, talking and talking, long after we've finished eating.
My fifteen year old still doesn't like the taste of alcohol, doesn't go to teenage parties yet and is happier with a J2O or Appeltiser.
I guess that the reason that we have "weaned" our DD17 on to alcohol (and will offer the same sort of opportunities to our younger when or if she shows any interest) is that we are sociable people who enjoy nice wine and good food with friends and think that it's on of life's pleasures, really. For us, it's a normal, acceptable and enjoyable part of life and so we are happy to include our kids. Just as we cook and eat with our kids and promote to them the sociable enjoyment of good food.
OP I would also be taken aback and feel uncomfortable to see 12 and 13 year olds are being given alcohol in the circumstances you describe although that may make me seem hypocritical since I let mine have a splash of Champagne on Christmas Day and New Year's Eve. I shall have to live with though, that because I'm happy with my decision.