Dimitry,

>> Please tell, where it is written that his original going forth was at the

age of 7 years?<<

Well it is just one of those known things. In the MN 61 comm. it states
that Rahula was seven when he asked for his inheritance. The Buddha was
striving for six years, and then spent his first rains in Varanasi. He
spent his second at Rajagaha, which was his seventh year from home, and from
there went to Kapilavatthu. This is related in the first section of the
Mahavagga.

Sean