Lata has always sounded more mature than Alka Yagnik and Kavita Krishnamurthy. It was noticeable earlier also. But perhaps people focused less because Alka, Kavita, Anuradha, Sadhana Sargam etc debuted in 80s when Lata was still going very strong and singing for main leads. So was Asha.
So they may have sounded more like contemporaries than seniors and juniors.
Though Alka slowly began to be preferred more to sing for younger girls like Neelam, Ayesha, Sonam, Manisha, Juhi, Madhuri, Raveena, Karishma in most films (barring Rajshri and YRF banners).
Subhash Ghai for eg had Lata sing for Madhuri in Ram Lakhan but chose Alka for Khalnayak. Indra Kumar took Anuradha Paudwal in Dil and Beta.
With Shreya and Sunidhi, the age and generation gap as well as style of music and age of actors had clearly changed which made singers sound more aged. Shreya's voice was so youthful and she started when she was just 17, which made even Alka, Kavita, Anuradha sound older.
Shreya and Sunidhi began to suit younger actors much better.
Anyway Sunidhi can sing new age kind of songs and taking her name here is not relevant as Lata never sang kind of songs Sunidhi does. Sunidhi has totally different style and genre. Sunidhi's voice suits contemporary music style way more. So no comparisons here.
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