Yes. that is their purpose, to still be valid.
Yes, that is the reason it is a 'Forever' stamp.
yes
A three cent stamp will be good forever for US postage. Every US stamp issued since 1861 is valid for postage.
Stamps are always and forever worth the denomination printed on the stamp. If you need a one-cent stamp to complete postage, and you have a one-cent stamp from 1933 - it is perfectly legal. As to what individual stamps are worth to stamp collectors - that is a completely different question.
It depends on the weight of the package or envelope. Letters weighing one ounce or less require a 49 cent stamp called a "forever" stamp.
No you don't, that's the whole purpose of the Forever stamps, they are valid for one ounce of First Class Postage at any rate.
Without knowing the date on the stamp, a description and condition, it is impossible to value a stamp.
This is the forever stamp -- its value as postage remains at the current first class rate. Even when the rate goes up, it can still be used without additional postage. Now it is the same as a 42 cent stamp-- after May 11, it becomes a 44-center.
There is a "Forever" stamp that commemorates the Islamic holidays Eid al-Fitr and Eid al-Adha. It can be purchased for 46 cents, though a new design has not been announced.
Yes. That's why it's called a "forever stamp". It will remain sufficient postage for a first-class letter indefinitely regardless of future postage increases.
One, plus a 21-cent extra-ounce stamp.
US stamps come in different denominations, but if you just asked for a stamp you would probably get a 44 -cent stamp which pays postage on a letter of up to one ounce. The "forever" stamps currently are considered to be 44-cent denomination.
No way of telling without knowing what country and the condition.