@ hajime gakusei
Where "thx" comment? :'(
(Not so angry, a bit disappointed)
1) "this building is 3 years old"
このビルは、築3年(chiku-san-nen)です。
築 for buiding/houses needs usual.
築 means "built from/from built year".
築 verb is 築く(kizu-ku) means "fabricate/build/make" for architecture mainly.
2) "I've lived here for 5 years"
ここに(or ここには)、 5年住んでいます。 (Not "de")
Using "de" or "ni" may understand a bit hard exactly.
3) "This car is 9 years old but I've owned it for 3 years".
この車は 9年落ちですが、私の物になってからは 3年です。
or
この車は 9年前の型式(orモデル)ですが、私が 3年前に(中古で)購入(or入手)したものです。
For vehicle also industrial products for working machines/tools need "model year = 年式(nen-siki)" with better.
In Japansee a lot of "counter word(s)" exist not understanding all so easier.
https://en.wikipedia.org/wiki/Japanese_counter_word4) Number writing
Kan-suuji/漢数字(Number of Kanji) using for vertical writing(縦書き/tate-gaki) mainly.
Sanyo-suuji/算用数字(Arabic figures/numerals) using for Horizontal writing(横書き/yoko-gaki) mainly.
But both have a lot of exception as word or by design/fashion(out of formal).
5)
Don't forget "、" if for exam/report or business mail/documents.
Jut btw family or friends(include online chat/forum) no need formal style no worry.
@ gfoulk
Just この車は、2005年式です。enough.
Or more compact (これ(は)、) 2005年式(です)。possible.