It's absolutely valid Russian and it'a pronounced "пятидесятиоднолетний". Those adjectives respond to the question "сколько летний" and more popular (though considered colloquial) form "скольки летний".
This class of adjectives can be used both with animate and inanimate nouns with the only exception - годовалый
(оr одногодовалый) and it’s derivatives - двухгодовалый, трёхгодовалый etc. - is used for 1-year child or animal, while "однолетний" is used in other cases.
The "годовалый"-form is interchangeably used with -летний
form for smaller numbers, so you can encounter both, say, "пятигодовалый" and "пятилетний" but you hardly ever see "тридцативухгодовалый".
The last thing I can add regarding this issue that I always missed the Polish noun-form, they, for instance, can say something like "zginął 34-latek", which is shorter. In Russian we have words like "трёхлетка" or "пятилетка" but they are basically used only for designating period of time.