So by far whatever bits and pieces were thrown here and there throughout the episodes, this is what I gathered -
Dev is 45 (Priya introducing him in his so-called Swayamvar recently). Also, he has taken up the mantle of taking care of his family since he was 15 (in some scene, Satyavati mentions this) and somewhere I also heard he has been taking care of his family for the past 20 years. So, the makers have been consistent about this so far.
Vidhi is 21. I think it was mentioned somewhere but don't remember the episode or scene. But since she is appearing for final year exams of her graduation degree, so 21 seems appropriate.
So the age gap is 24 years, which is quite huge. So, Dev falling in the age bracket of Vidhi's father (as mentioned by Vidhi's father whenever he meets Dev) is totally plausible. I am guessing Vidhi's father would be 5-6 years older than Dev at most. So, Vidhi calling him "Uncleji" in first few episodes was natural. Poor girl had to practice saying "Sir", whenever she saw him, spontaneously she would utter "Uncleji".
24