When is the earliest that asthma can be diagnosed in a new baby?

Hard to say. Most definitions of asthma depend on lung function testing, which is not commonly done before age 5 or 6. Children who wheeze prior to age three may or may not develop asthma that stretches further into childhood. In a baby, shortness of breath or wheezing that goes away with asthma inhalers is suggestive, but we often won't make a diagnosis of asthma until at least age 3 or later.
Difficult to answer. The diagnosis of asthma is based on several factors. In older children and adults, lung function testing can be performed. However, in infants, such testing is not possible. Therefore, the diagnosis of asthma in infants is based on symptoms such as cough, wheeze and shortness of breath. Other factors such as family history and coexistent diagnoses (i.e. Eczema or food allergy) can also help.
Cloudy picture. Infants get viral infections that trigger wheezing as infants (rsv & others) & are often treated with the same meds we use for asthma. Many have repeated episodes & come from families that have asthma. As many as 70% of these grow out of this by school age, so calling it asthma is a bit of a stretch. On the other hand, the worst kid I rxd over 18yrs for asthma spent part of his 1st yr in the icu.