Were the Istari already old men...?

[Belegorn]

Yes they arrived in the form of old men although they were full of vitality.

"They came therefore in the shape of Men, though they were never young and aged only slowly, and they had many powers of mind and hand." [UT, p. 409]

"for long they went about in simple guise, as it were of Men already old in years but hale in body" [p. 405]

and, "Men perceived that they did not die, but remained the same" although they "aged only by the cares and labours of many long years" [p. 406].

The main reason for this was so that they would not be so incredible to M-E inhabitants, "but coming in shapes weak and hunble".

So we have it that they are Maia who wear the shapes of old men so as not to come in a display of power, nor to subjugate any of the children of Eru. They were to rally the peoples against the enemy but not themselves go head to head with him. Although Gandalf did confront the Balrog and unleash his powers there.