jahrelang
for years
Continuing over a period of several years.
Sie haben jahrelang für dieses Haus gespart.
They saved for years for this house.

for years
Loading your saved state.
for years
Continuing over a period of several years.
Sie haben jahrelang für dieses Haus gespart.
They saved for years for this house.

The word is made up of 'Jahre' (years, plural of 'das Jahr') and 'lang' (long). So the meaning is literally 'the length of years'.
Die Renovierung des alten Hauses dauerte jahrelang.
The renovation of the old house took years.
'Jahrelang' (lasting for years) can also stand before a noun as an adjective. In that case, it has to be declined like any other adjective.
Nach einer jahrelangen Freundschaft haben sie geheiratet.
After a friendship that had lasted for years, they got married.
The structure '[unit of time] + lang' is very productive in German for describing periods of time. Other examples include 'tagelang' (for days), 'wochenlang' (for weeks), 'monatelang' (for months), or 'stundenlang' (for hours).
Wegen des Streiks mussten wir stundenlang am Bahnhof warten.
Because of the strike, we had to wait at the train station for hours.
year-old / year-long
'jährig' and 'jahrelang' can both express a duration of several years. The difference lies in precision and grammar. 'jährig' almost always appears with a number and describes exactly how many years something lasts, for example 'ein dreijähriger Kurs' (a three-year course). 'jahrelang' only means that something lasts over several years; the exact number is not important. It often suggests a long time. The words can be confused because both refer to duration. As a practical rule: exact number plus noun: 'jährig'; vague long duration: 'jahrelang'.

Mehmet hat jahrelang in derselben Werkstatt gearbeitet.
Mehmet worked in the same workshop for years.
Nach dem zweijährigen Kurs möchte Lisa als Erzieherin arbeiten.
After the two-year course, Lisa wants to work as an early childhood educator.