いいとしして
(in spite of) being old enough to know better
Also written いい歳して、いい齢して
Expression
いい年とししてそんな馬鹿ばかなまねはよせ。
At your age you should know better.
いい年としして、再婚さいこんしようとか本気ほんきで思おもってるの？
Are you seriously thinking about getting married again at your age?
いい年とししてラノベ読よんでる奴やつって普通ふつうに知恵遅ちえおくれの障害者しょうがいしゃだろうな。
Normally, a guy who reads light novels despite not being a kid anymore is a retard, don't you think?