I was just looking at how to translate ' 4 year old boy ' into Polish. I got 4 lata chłopiec, but was then horrified to see how this progressed through the different ages , seemingly without any logical pattern. Eg 2 letniego chłopca, 4 lata chłopca, 5 lat chłopiec, 9 letniego chłopca, 10 letni chłopiec, 14 letniego chłopca. itd. I can't see any pattern here. Also , why 23 letniego chłopca , but 23 letni mężczyzna and 23 lat kobieta and why 79 lat stary but 80 letni mężczyzna?

Ok, I think you're mixing cases, hence the confusion.

"4-year-old boy" will be "4-letni chłopiec" or "czteroletni chłopiec"

So generally speaking you will add "-letni"/"-letnia". While talking about 1-year-old, you can use the adjective "roczny"/"roczna"

Now, you will apply cases to -letni/-letnia endings, same way as you would to an adjective:

nom. 4-letni chłopiec
gen. 4-letniego chłopca
dat. 4-letniemu chłopcu
acc. 4-letniego chłopca
ins. 4-letnim chłopcem
loc. 4-letnim chłopcu
voc. 4-letni człopcze

I don't know where you got examples below, but let's break them down:
2 letniego chłopca -> that's gen. or acc.
4 lata chłopca -> that's incorrect, unless you use it like: "Znam chłopca 4 lata". It's an awkward phrase but people use it.
5 lat chłopiec -> incorrect
9 letniego chłopca -> gen. or acc.
10 letni chłopiec -> nom.
14 letniego chłopca -> gen. or acc.
23 letniego chłopca -> gen. or acc.
23 letni mężczyzna -> nom.
23 lat kobieta -> incorrect
79 lat stary -> incorrect. This is a direct claque from English
80 letni mężczyzna -> nom.

Does this help?

Hi Seraph. Thanks , yes it does help. I understand the Polish grammatical cases. However , I got these examples on the internet and they were very poorly presented, which confused the whole thing . Thanks for clearing it up for me.

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