If she were still alive, she would be 8 years old this month.
If she were still with us, we would be celebrating her 8th birthday this month.
仮定法なので、主語は she ですが was ではなく were を使うのが正しいです。
ーIf she were still alive, she would be 8 years old this month.
「もし彼女がまだ生きていたら、今月で８歳です」
ーIf she were still with us, we would be celebrating her 8th birthday this month.
「もし彼女がまだ私たちといたら（生きていたら）、私たちは今月彼女の８歳の誕生日を祝っているだろう」
to celebrate one's 8th birthday で「８歳の誕生日を祝う」
ご参考まで！
If she was still alive today, she would be ○ years old this month.
ご質問ありがとうございます。
・「If she was still alive today, she would be ○ years old this month. 」
＝彼女がもしまだ生きていたら今月で○歳です。
（例文）If she was still alive today, she would be 5 years old this month.
（訳）彼女がもしまだ生きていたら今月で5歳です。
（例文）My dog died. If she was still alive today, she would be 7 years old this month.
（訳）私の犬は死にました。彼女がもしまだ生きていたら今月で7歳です。
お役に立てれば嬉しいです。
Coco
カナダ 
日本 
