In countries that bitcoins that bitcoins are being regulated, bitcoins are being taxed when they are sometimes bought and being sold and when you are buying things at retailers with bitcoins, you are going to need to pay tax on the product that you are buying.

until today there is still many countries does not recognise bitcoin as legal currency, so how bitcoin became taxable. but, it is still not imposible because there is tax base on pernonal income. so, as long as bitcoin not converted to (real) currency like dinar, dolar, riyal, euro etc... it still possible to avoid the tax.

For some countries where bitcoin is not yet legal, bitcoin is not taxable yet. But for some other reason, withdrawing bitcoin or in order for you to use your bitcoin in other country, you need to convert it first.

Bitcoin is not invented to be taxable because it a digital currency.
But I think people should pay taxes for trading cryptocurrency if their country instead on that, I see nothing wrong with it.

It is taxable. Individuals pay taxes at rate lower than an ordinary taxable income if they have held bitcoins more than a year.

If bitcoin is to be launched in the public and be taught to colleges, i think the government will give taxes to bitcoin. But for me, i dont want bitcoin to have tax. It's better on how bitcoin is working now. As bitcoin is a digital currency, i think we cannot avoid having tax here.

It all depends on the particular national laws.

Well technically, it really depends on the country's laws that you are staying in. Normally, bitcoin itself is not taxable but mining is. A lawyer at our BSP (Central Bank) told us that bitcoin mining falls under the 'other income' bracket thus making it taxable. But I am pretty sure that most people earn their bitcoins through investments, trading, campaign signature or even gambling.
Also do note that the government will only tax something that they can control. Bitcoin in itself is decentralized, meaning no government entity can regulate it.

Yes, they are , but it depends on where are you located at the time that you want to withdraw your bitcoins. In the US, if i am not wrong ,you need to pay 15% of all your earnings, maybe a little bit more if you are not paying your full taxes.