Vertaling:Wij hebben al zestig jaar samengewoond.
I think that if you omit the al it could be interpreted as if these two people are not living together anymore, there's ambiguity in the message; while if you include it, it's clear that they still live together.
And samengewoond is the past participle of samenwonen, which is a compound verb. Remember that compound verbs (like the English phrasal verbs) are formed by combining a verb with an adverb, a preposition, or both. In Dutch, the adverb/preposition is always a prefix, which means it's 'glued' at the beginning of the verb. (Some compound verbs are formed by adding a noun, but these are less frequent -oh, and they are always inseparable).
Within compound verbs we can distinguish separable verbs and inseparable verbs. Samenwonen is separable.
For more information, please check the following link: http://dutchgrammar.com/en/?n=Verbs.Co01