-årsåldern

Etymology
From, "the X-years age."

Suffix

 * 1)  aged (at least and) around the specified number of years
 * 2)  aged the specified number of years
 * 3) the specified age
 * 1)  aged the specified number of years
 * 2) the specified age
 * 1) the specified age
 * 1) the specified age
 * 1) the specified age

Usage notes
"50-årsåldern" (as an arbitrary example) is often used to translate "fifties" and vice versa, but this can be inaccurate, as seen above. The intuition is closer to "early fifties." When it matters, "He is in his fifties" can more accurately be translated as "Han är i 50–60-årsåldern" or "Han är mellan 50 och 60." Another possibility is the informal "Han är 50-nånting" (He is 50-something). This might be considered an anglicism by some, but appears in newspapers.

"Late fifties" might require some rewriting like "nära sextio" (close to sixty), "närmare sextio än femtio" (closer to sixty than fifty), "närmade sig sextio" (was approaching sixty), or giving an exact age, though "sena femtioårsåldern" appears sporadically as well. Another rare but fairly idiomatic-sounding (and somewhat literary-sounding) alternative is "i sina sena femtio."