can you help me about several questens

1.How long has this bookship BEEN IN business?1990.
"in business" = 在开门.
2.I would like the pants IN the shop window please.
window 是有里面的,behind也可以
3.AT your age,i think the blue is better.
at your age = 在你的年龄; by your age = 到了你的年龄; for your age = 在你的年龄,这个使用来说你自己的 feature,不是说喜爱/讨厌的东西.
4.OF all the cars,this is the most expensive.