expression
for several years; over several years; several years standing
Used to describe a state, condition, or relationship that has continued for several years up to the present. Often appears in formal or written contexts.
数年来の友人と久しぶりに会った。
I met a friend of several years for the first time in a while.
この問題は数年来解決されていない。
This problem has not been resolved for several years.
数年間 simply indicates a span of several years, often used for completed periods or general duration, whereas 数年来 carries a nuance of 'since several years ago' and ongoing relevance.
Compound of 数年 (several years) and 来 (since, for the past). The 来 suffix indicates a period continuing from the past to the present.