I've never seen such a beautiful night since I became old enough to remember.
Literal
Awareness-attached since [topic-は] this-kind-of beautiful night [object-を] seen [experience-ことがない].
物心ついて以来 means 'since becoming aware of the world (as a child)' — 物心 is 'awareness of things,' and ~て以来 means 'ever since doing X.' ~たことがない expresses 'have never.' A poetic sentence with genuine emotional weight.