Any income is constructively received when you can actually spend it. Just like an office worker who earns $15 an hour but can only spend it when they actually receive a pay check (and is only taxed on the paycheck, not on hours earned but not paid yet), you record your income when the bitcoin is available to you in a form that you can use it.
However, be aware that mining bitcoin can potentially generate two separate taxable transactions.
First, you report and pay regular income tax on the value of new bitcoin that is credited to you using the value on the date it is created.
Then, unless you convert it to "real" money on the same day, you will also have a capital gain or loss transaction to report whenever you do convert the bitcoin to some "real" value (use it buy something or convert it to a recognized currency.) Bitcoin is treated like any other asset, stock or collectible. When you sell or cash out, you have either a capital gain or loss, that is taxed as regular income (short term gain, held one year or less) or lower long term capital gains rates (held more than 1 year.) Your "cost basis" is the creation value that you paid tax on originally. Then if you sell later for less, you may have a deductible loss, and if you sell later for more, you probably have a taxable capital gain. Because cryptocurrency exchanges are not currently required to issue 1099-B statements like a stock broker does, you will need your own accurate records of your purchases and sales. If you are audited and can't prove the price you paid, the IRS is likely to declare the entire amount to be a taxable gain.
