ok so after reading every point gayu mentioned.
good job gayu, bahut acche se point out ki tume ek ek baat.
that's why i like asking you doubts related to the show because u observe every aspect clearly.
and also considering points given by bagu,
i would se ki mallu and kallu never personally had any issue with age.
mallu's pov:
mallu ko darr tha ki yeh age factor ki negative impact kallu par pade. he was guilty for each and every sacrifices kallu did to save their marriage, like taking up all responsibilities, ghar ka, bacche ka, mallu ka. so he didn't want to stuff her life with more responsibilities by having husband-wife relation with her, cuz even though both were comfortable with each other such relation can bring in more responsibilities like having another baby or she getting distracted from her education and dreams.
and he might have also that ki (as gayu said) kallu abhi bacchi hai even though she is happy with her life now,when she grows up she might regrate the fact of marrying an elder man. etc.
bahar walon ke saamne he was never ashamed to tell anyone that she is his wife. usse bas kallu ki soch se fark padta hai. he didn't want ki kabhi ayis din aaye jab kallu unke beech ki age factor se pareshaan ho.
kallu's pov:
she happily accepted everything god decided for her and always respected and lived with the thought that mallu is her husband and just like any other wife she wanted to give him all the happiness as kallu keliye mallu is her forever wala rishta, so woh acche se poori tarah se rishta nibhana chahti thi taaki mallu ko kabhi koyi problem na ho that he married an 18 year old. that's why she always tried hard to fulfill all the responsibilities properly bahu,maa and patni thats why that SR wala scene, where she was even ready to follow sampu's ideology. and she tried to tie her hair into a bun thinking mallu likes such ladies 😆
bas dono yeh sochthe the ki ek dusre ko problem na ho iss wajah se. but the fact that both never openly told each other till rachit wala track is that they both loved each other and adored each other the same way.