Suppose some incredible breakthrough in medicine was made allowing people to live for 300+ years by means of genetic enhancement or some special medicine or medical procedure. Would thus be allowed by governments? I think many would oppose extending the lives of everyone currently alive for such a long period of time, and that the human population needs to be replenished for several reasons. I hope you can identify some of the downsides of implementing such a technology, and if it would be allowed by governments.
As @Magic-Mouse already stated, longevity will be expensive. Not necessary because the actual way of achieving it is expensive, but because it would be very valuable to some, and that would reflect in the price tag (unless it would be so simple that spreading the how-to alone would make it achievable for (almost) everybody). Assuming that most politicians like power, they would most likely like the power to live longer. That would prevent them from making it illegal. But it would also encourage them to make it hard to get (for others, not for themselves). That means: It would most likely not be illegal, but restricted. Maybe comparable to firearms laws in most countries.
A side note: Since overpopulation would be a problem if everybody lived four times as long as today, you could enter a limiting factor, for example by allowing longevity only for people without or with only one child, or something implicit like that the treatment for longevity makes people sterile (which would not prevent them from having many chilren before the treatment though).
In different stories, there is different kinds of artificial longevity. Either by surgery, medicaiton, replacement, cloning.
No matter how they do it and if it is permernent or just extension of life, there is always some common factor, money - Mostly it is some kind of noble or richman who have access to the extension formula.
Along with that - another common is that people living in those societies often have shortened lifespan, either due to bad medical care, crime, evil masters who make sure people work them selves to death and such.
This is due to the capacity of our planet. Without peoples ability to die, everything would get crowded and people would start panic. (Look at the Dr.Who Spinoff - torchwood, last season "Miracle Day" for reference).
As for the question in the title, i dont think it matters if it is legal or not, if it exists and you got money, you will have it.