My joints and muscles don't seem to move as well as I age.
ーI get stiffer as I get older.
「年をとるにつれ体が硬くなる」
to get stiffer で「（体などが）もっと硬くなる」
as I get older で「年をとるにつれ」
ーMy joints and muscles don't seem to move as well as I age.
「年をとるにつれ、関節と筋肉がうまく動かなくなるようだ」＝「硬くなるようだ」
joints で「関節」
muscles で「筋肉」
don't seem to move as well で「うまく動かないようだ」
ご参考まで！
カナダ 
