Originally, this article was to cover a different topic: the many terms in Japanese for various milestone birthdays (such as 白寿 for your 99th). During the research, I found such a comprehensive article, Japanese Words for Age, by Lloyd Vincent at NihonShock, that it seemed silly to do what would amount to a rewrite. It's really well done.
Instead, we'll look at some odd-sounding katakana words relating to ages.
アラサー, アラフォー and アラフィフ typically refer to a woman's age, but can also apply to men. If you've sounded them out and remain puzzled, see the expansion of アラサー: アラウンドサーティー, or "around 30". Once you see that, you can hear "around 40" and "around 50" in アラフォー and アラフィフ.
アラサー and "30-something" differ a bit: the first is around 28 to 32, while the second is usually 30 through 39.
Are these in common use? Are they rude to say? A tentative "no" to both, though it seems to depend on context. I wouldn't feel comfortable using them until I had more experience. But there are plenty of non-derogatory examples.
アラサー was apparently coined in 2006 by the fashion magazine GISELe, whose target readership is... women around 30. Other sites have adopted the term, such as 30kekkon.com, offering help with アラサー婚活 (finding a marriage partner at that age). In the same way 就活 is a 省略 (abbreviation) for 就職活動 (job hunting activity), 婚活 is short for 結婚活動 ("marriage [hunting] activity"). Good luck to any readers involved in either of these!
Andgirl magazine offers more advice in アラサーになって変わったことあるある4選, including: "男を顔で選ばなくなった" (you can [no longer] choose a man by his face).
The particular age ranges in the アラ series are up for discussion: many sites ask, and sometimes attempt to answer, questions like 流行語「アラフォー」は何歳から何歳まで?
Looking for love around 40 is a little more difficult, as a Machicon Ivery column explains: "アラフォー女性の婚活は厳しい" (for women around 40, the marriage hunt is tricky).
The 2010 Jennifer Aniston and Jason Bateman movie The Switch, Aniston's character, a single woman in her 30s, wants to have a baby, and has artificial insemination. The Japanese title for the movie is more 具体的 (concrete, definite) than the English title, but also advances the protagonist's age a bit: "アラフォー女子のベイビー・プラン", or "Around 40 Woman's Baby Plan".
If you're around 50, magazines still have you covered, as well as the Youtube channel yoriko makeup channel, with more than 350 videos, 27,000 subscribers, and 8 million views. On-topic titles include 【アラフィフ】, 【アラフィフ❣️】, and more recently 【50代・アラフィフ❣】.
Age 60 is the last of the common "アラ" terms, with a different form: アラカン alludes to "around 還暦", which completes one cycle of the 60-year calendar cycle. Chukyou TV in Nagoya hosts a weekly Arakan program (at 5:45 in the morning!) for retired people, both men and women, offering "セカンドライフをエンジョイするためのヒント" (tips for enjoying your Second Life). "Second Life" is life after retirement and one's second trip around the 60-year cycle.
There is one more. With its aging population and longevity (life expectancy of Japanese women is 87.5 years, second best in the world), Japan has the world's highest number of centenarians per capita. In raw numbers, only the U.S. has more. Although a search today brings up mostly dictionary sites, the term "アラハン" (around 100) may become more widely used as time goes on.
I hope you enjoyed this glimpse of the various "アラ" terms in use. See the links for more information!