Congrats on your new little one. She sure is a cutie!
You mentioned age... she couldn’t be one month old. Most budgies aren’t even weaned by that age, and it is not recommended for pet owners to buy a baby under 8-10 weeks old. Also, you mentioned a male at 3 months; a male’s cere at that age is no different than a male at any point before maturity.
Your girl is under 3-4 months of age from looking at the striped pattern on the head. When they start the first molt, you’ll see the stripes closest to the face start to disappear, and she will start getting a clear yellow “cap”.
She will greatly benefit from the addition of several toys, and perches made from natural branches, that you can buy online or at a pet shop. Also, as you already know, start keeping your eyes open for a larger cage. You can then use that one as a hospital cage or carrier.
Be sure to read our Stickies and Articles... you’ll find some essential information on keeping your girl healthy and happy. Diet tips, behavior, housing, healthcare, etc. You’ll also find answers to many common questions by reading the recommended material. If you still have questions we’re here.
Looking forward to hearing what you’ve named your girl