How old were you when you started dating more seriously?
I've never been NOT serious when I date. I've always been serious about about the person that I'm with, and serious about the relationship. I've done half-assed jobs on rare occasions, but I've never done a half-assed relationship because there's another person involved in it who happens to be important to myself.
A few of my ISTJ friends seem very focused and don't let men get in the way of their goals until they are ready. Like they will schedule them in to their lives. One is now at the settling down stage in her life, but the other has never been serious about guys, nor does she seem to want to be any time soon (which is fine).
I understand where that's coming from. We take our own futures pretty seriously; establishing a secure, satisfactory livelihood early on is something that we strive to do.
On the other hand, I personally think that finding love is something just as important in life. One has to 'work at it' just as rigorously, too - I had to learn to get up off my arse and actively keep searching for the right person (because she wouldn't just come along if you sit and wait... you have to find her), invest time and effort, undergo trial-and-error, and eventually learn to be better at dating. By better I mean how I conduct myself around the other person, communicate, express my feelings, 'quarrel well' and 'make up well' (for the lack of a better word).
Mutual love and respect is the basis of any romantic relationship, but in addition one would have to know these specifics in order to make it a strong, lasting one. Experience makes you recognize the right person when s/he comes, too. And you're freer to make mistakes the younger that you are, IMO, so it might be a good idea for anyone to keep the window of opportunity open, so to speak. Younger ISTJs should focus on establishing their careers, but not eliminate all possibilities of dating altogether. I for one have never regretted the time I spent crushing on someone, confessing love, dating, falling in love, falling out of love, having my heart broken, getting depressed, getting elated, courting, entering into a relationship etc etc. It made me grow as a person, and I've always been a sucker for love.