The passage that hints Oliver Twist was not expected to live very long is:
"But nature or inheritance had implanted a good sturdy spirit in Oliver's breast. It had had plenty of room to expand, thanks to the spare diet of the establishment; and **perhaps to this circumstance may be attributed his having any ninth birthday at all**."
This suggests that Oliver's survival to his ninth birthday might be considered surprising, given the conditions he endured, which were not conducive to a child's growth and well-being. The phrase "having any ninth birthday at all" implies that his reaching this age is an unexpected outcome.