I am(or you are) nearing middle age(old man).
I(or you) will reach the generation of middle age(old man) soon.
そろそろ:"sorosoro" means "soon".
おっさん:"o ssan" means "middle age(old man)". This is a slang and you should not use this word.
片足をつっこむ:"kataashi wo tukkomu" means "reach, join, come".
NANGI