@anon157181: I do not think you should ever date a previous client. I think I can see your perspective though: You and the previous client have a strong and meaningful connection, and who knows what the future may hold, so why not at least give it a chance?
In a successful counseling relationship, there is often a strong connection made. Many times, a client comes in to the relationship vulnerable. He or she looks to the therapist to be a stable relationship to rely on, during which he or she can receive encouragement and feel supported.
Personally, I think it is highly important for the client to experience having a healthy relationship. Maybe he or she has never had one, ever! And if the therapist and client are of the opposite genders, it is all the more important to have a healthy relationship that is solely platonic, because the client needs to know that non-sexual relationships are possible and can be healthy. Either way, you are the example of this relationship, which will help them in countless future relationships.
Feelings and emotions are very strong during therapy sessions, and the client can easily attribute strong and non-platonic feelings for their therapist in being so cared about and listened to (clinicians can as well, because they feel so respected, appreciated and helpful). As his or her therapist in the past, it was important back then for you to maintain the positive relationship and supportive role as the therapist and therapist alone.
I think it is important for the client to continue to have the memory and experience of a supportive therapist that was purely a support to them. To start dating the previous client would rob him or her of this (potentially one-in-a-lifetime) positive relationship experience that existed for the sole purpose of helping the client. The idea of dating the previous client may seem like you could continue to help the client and be that positive relationship, but would in fact change and corrupt the entire previous relationship to a different concept completely.
A foundation built on the relationship of a client and therapist dynamic does not bode well for a long and fulfilling relationship. One person's role was to only need help, while the other person's role was to only give help. Even though the dynamic changed with time, any resulting dating relationship cannot be healthy or beneficial for either party. I hope this helps and that you do not consider going through with it.