That not exactly true. While it may be argued that they are biologically immortal, this is just a matter of semantics. When cells age, the telomeres constantly get smaller, this is in simple terms what causes aging, wrinkling, etc. Lobsters constantly produce a chemical called telomerase that repairs and keeps the telomeres from shrinking. It is not wrong to say that they live forever, they just stay "young" forever. In the case of lobsters, "forever" is a relative term. It's not to say they can't live and grow forever theoretically, but that don't. Every so often a lobster will molt it's shell due to a lack of space, this is generally because of a couple of locational factors. Lobsters will continue to grow and molt as long as the locational and biological prerequisites are met, but, again, this can only go on forever in theory. It takes a lot of energy for a lobster to molt, amd once they reach a certain size, there is a physical inability and a lack of necessity to grow any further. Some adult lobsters never grow past the size of the ones you see in restaurants/stores, some stay even small due to the lack of necessity to grow, while others grow to sizes that are gargantuan for their species. Now on to the age thing. It is incredibly hard to determine the age of a lobster, as size really has no indication of age. Not only that, but when a lobster molts, it looses every bit of hard material that is part of it's being, that includes parts of it's gastrointestinal tract, and because of this, there is practically nothing on the animal to indicate it's age. There is one man (forget his name) who is thebhead of his field in studying crustaceous sea life and specifically aging lobsters, he has figured out a rather accurate way, but it's method eludes me at the moment. There are creatures on the planet that can grow and live forever, but they are far from lobsters, they are planarians, a species of flat worm (forget if it's all, or specific ones), but they secrete the same chemical telomerase to keep its telomeres from shrinking, but unlike the lobster, they can literally live forever. Even after being split directly down the middle, the 2 parts, repair their bodies and become 2 separate entities. A group of scientists who started with a single flat worm that was kept in captivity, now have sever thousand samples that have all come from the original, just by doing the arithmetic, the number of specemin grew exponentially.
TL:DR Lobsters aren't really immortal, and cannot continue to grow and live forever. This idea is generally just a bit of misunderstanding mixed with a bit of misinformation, the chemical telomerase essentially keeps them young for their entire lifespan, but doesn't keep them alive forever. A set of biological and locational criteria determine their size and how long they live. Flatworms are immortal however