Can my weight, age, or bad knee affect field sobriety results?

Yes. These are balance exercises, and anything affecting balance affects the result. Impairment is one item on that list, not the only one.

Age. Significant weight. Back, knee, and ankle problems. Inner-ear conditions. Neuropathy. Vertigo. Recent surgery. Prosthetics. Certain medications. Also exhaustion, and the ordinary physiological effect of adrenaline — being pulled over is frightening, and frightened people shake.

The training material directs officers to ask whether the person has any condition or injury affecting performance. Where that question is asked and the answer documented, it's on the record. Where it isn't, that's a gap worth developing — the officer administered a physical test without checking whether the person was physically able to perform it.

Medical records documenting a pre-existing condition can turn "he showed four clues" into something the jury understands differently.