Visitor insurance for Ahmedabad/Gandhinagar and visiting US or Europe from India

The cost of travel insurance will depend on (1) where you are traveling to (coming to the US is more expensive than going anywhere else), (2) your age, and usually (3) the length of your trip. Within those parameters, if you are traveling to the US, you can choose an economical fixed plan or a more expensive, but complete comprehensive plan. A fixed plan is a lower cost plan because it only covers a set amount for each medical expense. The downside is that you only know what the insurance company will pay on your behalf; you may owe a little or a lot -- it will all depend on how much, specifically, for each service, you are charged. Someone in their thirties will pay about $40 per month for a fixed plan or $60 for a comprehensive plan if traveling to the US. That same person would only pay $30 for a comprehensive plan to go to Europe.
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Yes, you can buy the travel insurance after starting the journey. However it is always recommended to purchase the travel insurance before departing. The claim will be invalid if the traveler attempts to buy the travel insurance and file a claim for an ailment after it occurred during the travel.

It depends on the company and the type of policy you are looking for. Many policies do not have a cut off age, but some do. Also, the main difference for older travelers is that the maximum of the policy available goes down with higher age. You may only find options in the $50,000 maximum or less, but you may still find something suitable for your situation. Find the best visitors medical coverage for older travelers visiting USA.