mahālakṣmī ca vidmahe viṣṇupatnī ca dhīmahi |
tan no lakṣmīḥ pracodayāt |

Would that make it older than the gayatri mantras at Maitrāyaṇī-Saṃhitā  2-9-1.  Does that make it the oldest gayatri mantra in the form 

... vidmahe
... dImahi
...pracodayAt |

Thanks,
Harry Spier