The age of imperialism is definitely over, and has been since the world powers were depleted in the wake of WWII. Museums can give people an interesting perspective on the harder forms of imperialism that was a part of history, even though people argue about continued cultural imperialism today in the world.
No, museums are not the only place to experience Imperialism anymore, because Imperialism, in the form of territorial relationships exist in a lot of places. Local politics can sometimes be imperialism of the powerful over the week. A school's administration over its students can be territorial, because their power is limited to a certain sphere. It is not just museums.
If you look in a history book you will see a lof of imperialsm. If you look at society today you will still see imperialism. Maybe not so much on a country level but on a group level within the country imperialism is still rampant and likely always will be.
Imperialism can be experienced in many places. It may not be able to see it in many places, but you can still experience it. There can even be reenactments that you can go to so that you may experience certain Imperial events. There is also many historical sites that you can get the feeling of Imperialism.
Museums are not the only place to experience Imperialism anymore. World travel would bring this to light for most people. The Middle East and Asia, for example, are full of imperialist countries, where a king or queen is the ultimate ruler. Most people will never visit these countries because most Americans do not travel outside of our own country.