Spirits essentially never gain anything from being in a bottle. There are a rare few bottlings that do have benefits from aging (mostly brandy-based), however those are difficult to find and these days only come from small distillers.
Since you use the word "spirit", which implies only those Liquors like Rum and Whiskey, the previous answers are correct. It's worth nothing that some Liqueres (like Bailey's Irish Cream) cannot keep as well over a long period of time. The specific reasons are varied, but largely have to do either with the use of a cream base or some cheap bottlings that are extremely high in sugar where some separation can happen.
Final issue, if the liquor can go bad once the bottle is opened. In some cases, like most Vodka, it almost absolutely won't go bad. Many Tequilas will go bad after exposure to air, it just takes quite a while. Be careful with anything that has been made from fresh fruits and high sugar content, as those too pose some risk of going bad. Vermouth also tends to get questionable after a long enough exposure.
it ages in the distillery, in a barrel. Whether 8, 10, 13, 15, or 18 years, it does get smoother in the barrel, but stops aging when bottled. Wine is the only alcohol which continues aging in the bottle.