At what age should a young man start dressing like an grown up?
You might also look at the question from the other side - at what age does it make sense to start making notable investments in clothing. I would guess 18, based on when people reach their full height. Aside from that, there is no point not having nice clothes for children at the age of 5 and beyond, assuming you can get them for good deals or are hand-me-downs.
Condolences on the death in the family, always a tough time.
I'd say it depends on each family. If it were a lower income family where money is tight, it might not be a smart choice to buy suits for children esp since they are going to outgrow it soon. When I was growing up I never needed a suit till I was 20.
At the same time some kids go to many events or out to dinner where a suit would be needed, if that's the case I'd buy them one.
I'd say around 20-21 would be when I would really invest in my kids wardrobe instead of buying them beaters. They've stopped growing, and are entering like 3rd year in college, so they shouldn't be gaining too much more partying weight and are going to start interviews in the near future
Since the navy sports coat is so common, you can probably find one at a thrift shop for about $3.
I'm sorry to read about your mother...damn...At 13, he's probably going to grow out of whatever she buys him pretty quickly, so I wouldn't go crazy. I'm not a big fan of navy blazers, but it's definitely an option. Realistically, he could wear dark pants and a dark sweater/shirt without seeming out of place.
As far as the title question goes, "dressing like a grown up" has a lot to do with your environment. Some men never need a blue blazer, let alone a suit, and that's perfectly fine. I don't think that the "Styleforum approved" looks are necessarily more "grown up" than the others are. You have to be aware of your environment.
Regarding the clothing question, I didn't purchase anything worthwhile until I'd reached a comfortable weight (and height). With my own children, I don't plan on purchasing them anything non-disposable until they've reached (and maintained for a time) their adult height and weight. In my opinion if they're going to inevitably outgrow it, it severely limits how much I'd be willing to spend.
Thrift or ebay for a child's blazer if it's an important enough issue - otherwise a dark sweater and slacks should be fine.
I really think that you are making far too much of this. The body of a 12/3 year old is still growing and developing and will not reach any form of stability until 16+ - and will even then go on changing.
Much depends upon his social standing but it is not unreasonable for him NOT to have the clothes that you desire at this age. I rather fear that your sister is more concerned at a personal desire to see mourners in some form of uniform which she considers gives the best appearance.
Let him come if appropriate, wear a shirt and tie and leave it at that and do not make any deal of it.
Funeral attendance by people of any age should not be a clothes show and no one should be deterred simply because they do not own that which some people misguidedly think obligatory.
Edited by GBR - 1/22/13 at 2:57am
It's a good question. What is meant by the term "grown up" in relation to clothes? I still see men in their 70s wearing jeans and T shirts and it suits them. Personally I wear less jeans and more trousers these days because I think they suit me better. If I could get away with wearing jeans I'd probably wear them more often. Image is everything.