חינוך לנער על פי דרכו גם כי יזקין לא יסור ממנו
The Noam Elimelech (פרשת בא) understands this teaching like so: Normally, one's acheivements in old age are caught up and distracted in [the] embarrassment [of rectifying and undoing all of the folly] of one's youth. Chazal teach us, that if one's youth is begun along the correct path, old age will be a time of great achievement because one won't be embarrassed with past ways and transgressions.
The Tanya instead points out that if the teaching is according to 'his way' and not according to the way of 'ultimate truth' what good is it that he will not stray from this way in his old age, isn't it proper that he should stray from it and find the way of 'ultimate truth'? In truth, this way, of proper awe and love of God is what will keep him on track even when he is beset upon by great difficulty. When he is not overly exerted, he will adhere to the way of 'ultimate truth' but when he falls, he will still cling to 'his way' and never fall in his observance of God's mitzwoth.
To me, it seems the Tanya hold true to the Beinoni Torah, reminding us that even if we do achieve the level of Tzaddik, we can't rely on it, and if we fall, it is the way of the Beinoni that will keep us on track and allow us to return to the level of Tzaddik.
The Noam Elimelech seems to take the Tanya teaching for granted, more concerned with once the Tzaddik has overcome the challenge of the fall before the rise, (ירידה לצורך עליה) he won't be able to stand before HaShem because of the intensity of embarassment only a Tzaddik could feel towards his past transgressions.
What we can learn from this: What chance do we have as parents if we don't learn hassidut? :)