21世紀にも親しみを覚えるようになって久しい、そんなある年。
I would translate 21世紀にも親しみを覚えるようになって久しい into "It has been a long time since I have finally become familiar with the 21ST century too.", but I'm not sure about how to interpret そんなある年 in this sentence.
I found 某年 on dictionary.goo.ne.jp/ with the following explanation:
某年: ある年。その年が不明の場合、また明示したくない場合に用いる。「某年某月」
I was wondering if そんなある年 in the sentence was referring to 'in a certain year' or 'at a certain age'.
Thank you in advance for your kind guidance.
21世紀にも親しみを覚えるようになって久しい、そんなある年。
I was wondering if そんなある年 in the sentence was referring to 'in a certain year' or 'at a certain age'.
The 年 is "year", not "age". ある年 here is "(in) one year".
そんな ("such") modifies ある年. It refers to 21世紀にも親しみを覚えるようになって久しい.
You could rephrase the line with no そんな, as in:
［21世紀にも親しみを覚えるようになって久しい］ある年。
Literally, "(In) one year, when it's been a while since we've been familiar with the 21st century"
With そんな, it'd be more like:
「21世紀にも親しみを覚えるようになって久しい、」（←そんな）ある年。
Literally: "It's been a while since we've been familiar with the 21st century. (In) one such year"
This usage of そんな sounds quite literary and is often used in literary works such as 小説, 随筆, 物語, rather than in daily, casual conversation.
Regarding the も, it's not for enumeration nor emphasis, but light exclamation/admiration, 軽い詠嘆の『も』. Please refer to:
